Meet John O'Connor: environmentalist, millionaire, candidate for Congress. There's a good chance you've seen his ads on TV, the third of which hit the airwaves this week.

O'Connor bears similarities to several of his opponents in the race to succeed retiring Representative Joe Kennedy (D-Brighton), but he is utterly unlike any one of them. He is, for instance, as big a jock as former Boston mayor Ray Flynn. He throws off as much kinetic energy as former talk-radio host Marjorie Clapprood. He has as much hunger for the details of policy as venture capitalist and former think-tank chairman Chris Gabrieli.

The net result is a kind of locker-room version of Ralph Nader, complete with a booming baritone and a love of vernacular that surely makes him the only candidate prone to shouting out "You da man!" John O'Connor is the kind of guy who can explain the effect of high-sulfur-oil burning on asthma rates and, in the next breath, brag with a cocksure grin that he's got "the best jump shot in the race."

For instance, right now O'Connor is riding shotgun in his Range Rover, en route to a meet-and-greet at a Cambridge tavern, blasting the song "Ghetto Superstar" from the Bulworth soundtrack, his head bobbing up and down to the beat. When the song is over, O'Connor jokes that he could be a version of the movie's rapping white politician himself. One thing leads to the next, and before you know it . . .

Moments later, a decidedly more serious O'Connor is at the Plough and Stars pub near Central Square, engaged in a lively bit of barstool democracy over the subject of his rap: his effort to repeal a recently enacted restructuring of the state's electric power industry. At the bar, O'Connor encounters a man who has actually read a book O'Connor cowrote on the subject of renewable energy. Even some of the beer-bellied working stiffs gather around to hear O'Connor and his new acquaintance trade fabulously arcane details as they debate how best to promote alternative energy. O'Connor even names the exact figure the federal government spends on solar energy.

These are the two John O'Connors: one a freewheeling urban jock, the other an impassioned, power-to-the people environmentalist. Now O'Connor is trying to stitch those two personas together in his bid to represent the Massachusetts Eighth Congressional District, which encompasses Belmont, Cambridge, Charlestown, Chelsea, Somerville, Watertown, and half of Boston.

O'Connor, 43, is pitching himself as a long-time foe of corporate America's greatest villains -- polluters, tobacco companies, pharmaceutical giants. As someone who can be counted on to keep the little guy from getting trampled in Washington and to preserve democracy from corporate onslaught. Ralph Nader himself testifies in one O'Connor brochure that "you can count on him to represent workers, small taxpayers, and consumers against the `big business' lobbies."

So far, O'Connor has been only a minor presence in polls. A Boston Globe/WBZ-TV survey conducted at the beginning of the campaign showed O'Connor at just 4 percent, well behind the front-running Ray Flynn, with 21 percent. More-recent surveys appear to show roughly the same results. But there is reason to believe that O'Connor will be in the thick of things in the race's chaotic final days.

Reason number one is, quite simply, money. O'Connor acquired a fortune -- now between $5 million and $25 million, according to his financial disclosure reports -- when he married Star Market heiress Carolyn Mugar, whose brother has long bankrolled Boston's Fourth of July fireworks extravaganza on the Esplanade. Money means advertising, and O'Connor's ad machine is heating up. This week, he began airing his second spot in the past two weeks -- an attack on Republican cuts in Medicare spending.

Second, recent events may give O'Connor the opportunity to distinguish himself from Chris Gabrieli. As the race's two multimillionaires, Gabrieli and O'Connor have routinely been paired in the media's attempts to cover the jumbled, 13-candidate field. To date, O'Connor has been somewhat overshadowed by Gabrieli's $1.5 million-plus spending spree. But Gabrieli appears to have gotten little poll movement for his money, and some observers now wonder whether his moment of opportunity has passed. Worse, Gabrieli may have been badly damaged by recent reports suggesting that a medical software company he cofounded in the 1980s (and from which he profited vastly) promoted just the sort of managed-care horrors he has denounced in a series of TV and newspaper ads. With Gabrieli at least temporarily limping, people may now take a closer look at O'Connor.

Actually, what O'Connor probably wants most is to shake off Gabrieli comparisons altogether. Says MassPIRG legislative director Rob Sargent, a sometime political ally of O'Connor: "John must be frustrated that he's labeled a millionaire businessman first -- and then as an afterthought they say he's an environmentalist -- when in fact he comes out of a long tradition of grassroots organizing and environmental activism."

O'Connor has indeed built an impressive record of activism -- from union organizing in the 1970s to pushing major environmental legislation in the 1980s to nurturing small enviro-businesses and doing battle with the state legislature and the electric utility industry in the 1990s. But then again, it's largely because of his millions that a man with no prior experience in elective office is being taken seriously in this campaign.

John O'Connor keeps a framed slogan in his office that neatly sums up his philosophy: THE FUN IS IN THE FIGHT. From the basketball court to the State House, O'Connor obviously relishes the competition of a good battle. But the experience he credits with turning him into a fighter was anything but fun.

O'Connor grew up in Stratford, Connecticut, an industrial town then home to Raybestos, one of the largest asbestos makers in the US. The company was a community anchor; O'Connor's Little League team was even called the Raybestos Cardinals.

It wasn't until O'Connor's sophomore year at Clark University, in Worcester, that he learned about Raybestos's possible responsibility for high cancer rates in his neighborhood, and perhaps for the cancer death of a Little League teammate. Even worse, O'Connor learned that the company knew about the harmful effects of its product -- and still let the kids play on a baseball diamond atop an asbestos dump.

"When I discovered that the company was hurting us and never telling us, my whole world-view was shattered," O'Connor says. Today, the former Raybestos plant is the site of a massive federal environmental cleanup. The experience, O'Connor said in his May 30 campaign kickoff speech, "made me vow that I wasn't going to sit idly by and watch as innocent people die unnecessary, horrible, preventable deaths."

From then on, O'Connor was a man with a mission. After college he organized low- income Worcester neighborhoods for the Volunteers in Service to America (VISTA) program. In 1983 O'Connor founded a grassroots environmental movement, the National Toxics Campaign, that ultimately culminated in the 1986 passage of Congress's landmark Superfund environmental cleanup law.

Throughout that time, O'Connor was helping to fashion a new brand of what you might call urban-populist environmentalism.

"He pioneered some of the key urban toxics issues, when previously environmentalism had been preservation- and middle class-oriented," says Lee Ketelsen, New England director of Clean Water Action, a national environmental group that recently endorsed O'Connor.

Since 1991, O'Connor has run Greenworks, a company that helps "incubate" fledgling environmental businesses. But founding a business didn't mean settling down.

Indeed, even as he runs his own campaign for Congress, O'Connor is locked in a battle with the state's electric utilities over the fate of a sweeping utility deregulation law passed by the legislature last fall. O'Connor fought to block passage of the law, which Ralph Nader called "the biggest consumer ripoff in Massachusetts history," spending hundreds of thousands from his own pocket on ads arguing that deregulation should have included stronger environmental protections and delivered billions more in savings for consumers.

Critics say O'Connor has exaggerated the evils of deregulation, failed to propose a detailed alternative of his own, and foolishly ignored the threat of economic chaos if the law is repealed now. Even admirers think O'Connor may sometimes become too caught up in a black-and-white world-view, raising the question of whether he has the pragmatic instincts to deliver for his district on Capitol Hill.

Undaunted by deregulation's passage, O'Connor is now fighting to repeal it with a November ballot initiative -- an effort that cleared its final hurdle last week when the state's Supreme Judicial Court struck down a vehement industry challenge.

Which means O'Connor is not only running his own campaign, he's also managing the repeal effort -- and he hints that they're not unrelated.

"One way to get a free microphone is to run for Congress," he says with characteristic frankness. "This is a way -- I hate to say it -- to keep working on a issue that I care deeply about.

"We'll see what happens on September 15," O'Connor says. "But I'm the only guy who wakes up the next day and has to go back to work fighting the giant special interests."

The holy war against "giant special interests" is the central theme of O'Connor's campaign. His television ads have already blasted corporate polluters and pharmaceutical giants, whom he attacks for making lavish profits from drugs developed with government subsidies.

Of course, the environment remains O'Connor's favorite topic. Don't get him started on pollution prevention, for instance, unless you're prepared to learn why it's "in the interest of every dry cleaner to buy a solvent recovery still," how the still recondenses gas, and why reduced perclorethylene use is a good thing.

But when discussing issues outside the realm of energy and the environment, O'Connor seems less energized, even if his grasp of details is still formidable. On his top three nonenvironmental priorities, he merely echoes his fellow candidates: he comes out against privatizing Social Security and calls for universal health insurance and an infusion of new funds into the schools. But then Social Security, education, and health care lack clear villain figures to rail against.

Perhaps it's just a matter of practice. After all, most of O'Connor's opponents have spent years -- in some cases, a couple of decades -- selling themselves to the public. That helps explain why political handicappers still don't rank O'Connor in the race's top tier with the likes of Flynn, Clapprood, Somerville mayor Mike Capuano, and former Watertown state senator George Bachrach.

But the power of advertising may yet change that. Some may argue that big spending has gotten Chris Gabrieli nowhere, but O'Connor has a well-honed field operation to go along with his paid media slots -- including the service of nearly 30 volunteers being trained at an innovative "campaign school" run out of his Cambridge headquarters.

O'Connor campaign manager and long-time liberal activist Jim Braude argues that O'Connor's success will ultimately depend on old-fashioned get-out-the-vote drives and door-to-door campaigning. "Not only does John understand person-to-person contact," Braude says, "it's the only way he knows how to do anything."

But that may be a curse as well as a blessing. There are indeed two John O'Connors in this campaign. The laid-back O'Connor of barstools and basketball courts leaves indelible impressions and makes instant converts of strangers. The other, a more serious figure, speaks gravely in television ads or talks dry policy at candidates' forums. And in those settings, he is often flat and uninspiring.

Watching O'Connor's TV spots, says political analyst Kevin Sowyrda, "makes you want to hit the clicker the same way as when a Tylenol ad comes on."

Would that every voter could have seen John O'Connor at the Corporal Burns Playground in Cambridge one recent evening pursuing his truest passion: basketball.

Many white people would have been intimidated by the young black men hanging out in the shadows just off the courts. But O'Connor easily cracked wise with these friends from his pickup and league games over the years.

At one point, O'Connor was greeted by a black boy in his early teens who wandered onto the court -- wearing a JOHN O'CONNOR FOR CONGRESS T-shirt, no less. It seemed too good to be true. But asked how it was that a young boy just happened to be wearing a campaign slogan, a marveling aide offered a convincing explanation: O'Connor had put the kid's older brother through college.

John O'Connor may yet surge into the top tier of candidates in this race. It won't be easy. But if the election were decided on moments like these, he would be all but invincible.