Sen. Scott Brown (R-Mass.) shakes hands with his Democratic challenger, Elizabeth Warren, before their first debate on Thursday. Until recently, the race had been notably civil. (Michael Dwyer/AP)

The most closely watched Senate race in the country has taken a sharp turn off the high road.

As Sen. Scott Brown (R-Mass.) and challenger Elizabeth Warren (D) enter the final phase of their exceedingly tight race, each is seeking to undermine the other on the very traits that had been considered their greatest political strengths: his independence and her character.

Brown is suggesting that the woman who made a national reputation as a fierce advocate for the middle class and consumers is a phony.

Warren, meanwhile, is urging Massachusetts voters to look beyond their affection for Brown to consider the votes he has cast and the national consequences of an election that could help return the Senate to Republican control.

Brown on Monday launched the most brutal salvo yet in a campaign that until recently was notable for its civility. His new television ad highlights the controversy over Warren’s unproved claim that she is of Native American heritage. It also raises the possibility — also unproved, and denied by those involved in hiring her — that she claimed minority status for professional advancement.

The 30-second spot resurrects what Republicans call the “Fauxcahontas” flap, which had died down since erupting last spring. At the end of the ad, an interviewer asks Warren whether anything else will come out about her, and the Harvard Law School professor replies, laughing, “You know, I don’t think so, but who knows?”

It amplifies a larger message that Brown has been pounding over and over in recent days, as he has also questioned the size of her Harvard salary and her past work on behalf of an insurance company involved in an asbestos suit.

“The true Elizabeth Warren is coming out and will continue to come out,” Brown said at an appearance Saturday in South Boston, the only part of this deeply Democratic city to support him when he won his Senate seat in a special election two years ago.

Though theirs is the most expensive Senate race in the country, with more than $53 million raised so far, the two candidates made a pact in January to disavow advertising by outside groups. That had helped keep the race more positive and restrained than many others, as Brown sought to maintain his nice-guy image and Warren to tamp down hers as a scold.

Warren launched the first negative ads of the campaign this month, at a time when some Democrats — including former governor Michael Dukakis, the 1988 presidential nominee — were criticizing the candidate and her campaign for failing to connect.

It was a gentle jab at first.

“Scott Brown’s not a bad guy. He doesn’t always vote the wrong way,” Warren said in one of those ads. “But too often, on things that really matter, he’s not with you.” She cited Brown’s opposition to President Obama’s jobs bills, his vote against imposing higher taxes on millionaires and his support for subsidies to oil companies.

But in a fundraising e-mail sent after his new ad appeared on Monday, she was blunter: “We know where Scott Brown stands — and it’s not with the people of Massachusetts. It’s with big money and his Republican buddies.”

The latest polls show the race to be a tight one, with the candidates trading narrow leads from one survey to the next. Ordinarily, a Democrat would hold a significant advantage, given that this is a presidential election year when Massachusetts voters are expected to come out in force to reelect Obama. The president leads Mitt Romney by more than 20 percentage points in recent polls.

Brown stunned the state’s Democratic political establishment two years ago, when he won the special election to fill the seat left vacant by the death of senator Edward M. Kennedy (D).

In his reelection bid, Brown has been emphasizing a willingness to work across party lines. One of his ads showed Obama congratulating him with a “good job” at a White House signing ceremony for legislation to curb insider trading by members of Congress.

“Brown has a lot of support in this neighborhood,” said John Stenson, owner of the Eire Pub in Dorchester, locally famous as a stop for politicians, including President Ronald Reagan in 1983 and Bill Clinton in 1992.

While Dorchester is considered Democratic territory, Stenson said, many there relate to Brown’s working-class roots and his independence.

“They like the way he came up,” Stenson said. “He’s not a silver bullet for either side. He seems to be an independent thinker.”

Brown was there on Saturday afternoon, conducting interviews from behind the bar while his wife, Gail Huff, a longtime Boston television reporter, was downing a pastrami sandwich and a pint of Guinness in a booth in the corner.

The senator invariably refers to his opponent as “Professor Warren,” a none-too-subtle suggestion of elitism. He also cites her six-figure salary as an example of why college costs are so high.

But Warren has her own up-by-her-bootstraps story, as the daughter of a man who once worked as a janitor in Oklahoma.

Her supporters say they are unfazed — and indeed, impressed — by the elite world in which she has established herself.

“She’s a professor. Do you get any better than that?” asked Adelina Tiberio, waiting to meet Warren at a festival in Watertown on Saturday. Tiberio made it only as far as the fourth grade before emigrating from Italy more than half a century ago, but she said her son got a PhD from Cornell.

Warren has begun bringing in some powerful reinforcements. On Friday, for instance, Boston’s Democratic Mayor Thomas Menino stepped off the sidelines to endorse Warren and promised that he would put his political machine to work on her behalf.

And on Monday, AFL-CIO President Richard L. Trumka gave a speech in Boston in which he pleaded with his membership, especially men, to get beyond what he suggested is lingering sexism and get behind Warren.

We have a problem because some voters — and let me be perfectly honest, I’m talking about voters who look just like me — have not stood up beside Elizabeth Warren to support her,” Trumka said. “There may be dozens of good reasons for us to vote for her, but it’s crazy not to vote for her because she’s a woman, or because she’s a college professor, or for any other superficial reason.”

“Do we want a buddy who’ll pat us on the back? Who wears a Bruins jersey with the boys?” Trumka added. “Or a leader who will fight for our right to form unions and bargain for a better life?”

In 2008, exit polls indicated that nearly a third of Massachusetts voters lived in a union household. But last week, an internal AFL-CIO poll indicated that they are no more likely than other voters to support Warren.

Though Warren can come off as stilted in speeches and on television, she is an energetic campaigner. In settings such as the Watertown fair, where she made her way through the crowd dispensing hugs to the adults and pinkie swears to the kids, she can seem more personable.

It had been little more than a day since her first televised debate with Brown, and many of her supporters were offering advice.

Asked what they were telling her, Warren said: “Gee, I’ve got to stick with the G-rated stuff. People say, go out and — I’m going to put this diplomatically — hold him to his record. The other piece of advice is talk about control of the Senate.”

In other words, take off the gloves — a message that both candidates appear to be heeding.