With Harvard Law Professor Elizabeth Warren gaining ground in the Massachusetts Senate race, US Senator Scott Brown's campaign decided there was nowhere to go but negative. Brown has blasted Warren for months, charging that she claimed Native American heritage to take advantage of university affirmative action employment policies. In the first debate between the two, Brown went so far as to say that Warren did not look Indian.

But his campaign finally overreached. At a recent Boston campaign event, a group of Brown supporters, including some members of his Senate staff, taunted Warren's backers with tomahawk chops and pseudo war chants. Suffice to say that such displays don't go down well with Native Americans.

Brown's first apology wasn't enough for the Cherokee Nation. "A campaign that would allow and condone such offensive and racist behavior must be called to task for their actions," Principal Chief Bill John Baker said in a statement. Brown was finally forced to come up with a stronger mea culpa and a zero tolerance policy on any future stunts.

The no-more-Mr-Nice-Guy strategy is fraught with perils for Brown. He won his Senate seat, in part, thanks to his carefully crafted persona as a genial, regular Joe in an old barn coat who cruises around the state in a pickup truck. Even long-time Boston Mayor Thomas Menino, a Democratic warhorse if there ever was one, counts Brown as a friend and waited as long as politically possible before endorsing Warren.

The feisty Warren is easygoing and personable on the campaign trail. But she's opted against being either warm and fuzzy or too in-your-face where it counts for most voters: on television. Her response to the heritage controversy has been muddled and only recently has she come out with a more robust response.

Dial up the wayback machine and the personable Brown might well have been making short work of Warren, as he did with state Attorney General Martha Coakley in the 2010 special election to fill the late Senator Ted Kennedy's seat. But in this Senate race, personal appeal is bumping up hard against the realities of party politics in a presidential election year. According to Alan Wolfe, a Boston College political science professor:

"If we lived in a world in which we vote for the candidate [as an individual], you couldn't lose as a middle class person by voting for either, but we don't live in that world. Party … matters right now more than the person, and the parties right now have very different approaches to the middle class."

The two candidates break even on the crucial issue of middle class "cred". Warren often describes herself as growing up on the "ragged edge of the middle class". She worked as a waitress as a young teenager, after her father had had a heart attack. Her self-description could just as easily also apply to Brown, whose working mother briefly relied on welfare.

So, Brown's strategy is to keep Warren off her game, which runs on her deep knowledge of the economy, consumer affairs, and Wall Street, lest she get more traction in turning the race into a referendum on the national Republican party and Brown's Senate record.

Four of five recent state-wide polls showed Warren edging out in front of Brown in what remains a very close race. Warren picked up her Democratic national convention bounce and ran with it. She hits all the right notes for her Democratic base on Massachusetts middle-class worries like unaffordable mortgages, student loan and credit card debt and bad guys like "Big Oil" and "millionaires and billionaires". For his part, Brown passed up a Republican national convention speaking slot – a move that deliberately distanced him from Mitt Romney, the state's former governor and his political mentor.

In the Senate, Brown has little room to maneuver on controversial issues like tax policy that are central to Republican orthodoxy. Few Massachusetts likely voters believe that he would raise tax rates on the wealthy Americans. Indeed, he's opposed the "Buffett rule" and lifting Bush era tax cuts on higher-income earners while preserving them for workers making less than $250,000.

Bay State voters are cool toward their former governor – President Obama is expected to trounce Romney easily here – and cooler still toward the conservative Republicans who dominate the national party. Yet, a strong Democratic turnout doesn't guarantee victory for Warren in a state where independents comprise more than 50% of the electorate and historically have few qualms about voting for moderate Republicans like Brown.

Which is why the tomahawk/chant episode could be a pivotal moment. It recalls a ghost of campaigns past: the 2006 Massachusetts gubernatorial race. Kerry Healey, the Republican candidate and Romney's lieutenant governor, deployed an ad loaded with powerful racial overtones against the Deval Patrick, the African-American political newcomer. The tactic backfired badly: Healey never recovered and lost to Patrick in a rout. He is now in his midway through his second term; she advises the Romney campaign.

Much is riding on the mood of the Massachusetts electorate. Do voters believe that Brown is still an affable guy who understands their issues better and can make himself heard in a politically polarized capital? Or is the Harvard law school maverick and consumer protection champion the best person to send to Washington?

Clearly, Brown has opted to keep voters focused on his view of Warren's personal shortcomings. That could be a costly gamble for a nice guy.