BOSTON (AP) -- Republican U.S. Sen. Scott Brown and his chief Democratic rival, Elizabeth Warren, have vowed to fight attack ads on television, radio and the Internet, but in their emailed appeals to supporters, the two routinely portray each other in the harshest light possible.
Brown and his campaign manager, Jim Barnett, have called Warren a "far-left ideologue," "ardent leftist ," "wealthy one-percenter," "rock-throwing street fighter" and "elitist hypocrite" who's running "a campaign based on self-righteousness and moral superiority."
Warren and her campaign manager, Mindy Myers, have also used tough rhetoric in their fundraising emails, describing Brown as "partisan a Republican as they come" and a politician who sides with "the right wing of his party, against the people of Massachusetts."
In part, the rougher tone reflects the partisan dynamics of political fundraising, where firing up one's base inevitably means taking shots at one's opponent.
But in Massachusetts, there's an added twist -- an unusual agreement struck by Brown and Warren to keep third party groups from running attack ads during the campaign.
That means if there's any mudslinging in the works, it will have to come directly from the candidates and their campaigns.
The nearly daily email missives show how Brown and Warren are trying to find ways to attack each other without tarnishing their own campaigns.
One of most recent sparring matches erupted over a fundraising appeal from Brown in which he casts the Massachusetts Senate race in the context of the presidential contest. Brown supports fellow Massachusetts Republican Mitt Romney.
"I know there are several other GOP campaigns to support, but this race is THE battleground for the United States Senate -- the only sure hedge to a potential second term for President Obama," Brown wrote.
Democrats, including Warren's campaign, were quick to note that Brown's message appeared to clash with his recent public efforts to find common ground with Obama.
When Obama called on Congress during his State of the Union speech this year to "send me a bill that bans insider trading by members of Congress," Brown, who sponsored the bill, was quick to highlight the endorsement. Brown's office put out a press release that included a link to a video showing a brief exchange between Brown and Obama during which Brown urged Obama to push Majority Leader Harry Reid to release the bill and Obama said, "I'm going to tell him to get it done."
Brown also regularly boasts of being one of the most bipartisan senators in the Senate as he appeals to independent and conservative Democratic voters in Massachusetts, where Republicans account for just 11 percent of the electorate.
"On TV, Scott Brown says that he's an independent voice in the Senate. But that's not the message he sends to his campaign supporters," Myers said in a follow-up fundraising email to Warren supporters. "In fact, when the cameras aren't rolling, you see Scott Brown's true colors -- and he's as partisan a Republican as they come."
Despite his recent efforts to show solidarity with Obama, who remains popular in Massachusetts, Brown won his Senate seat by vowing to be the 41st vote again Obama's health care bill.
For Brown, a key to his re-election is maintaining his likeability among voters -- a likeability that could be threatened if he's forced to attack Warren as Election Day approaches.
It's not the first time Brown has tried to appeal to voters by eschewing the rougher edges of politics.
In the 2010 special election that netted him the Senate seat held for nearly half a century by the late Democratic Sen. Edward Kennedy, Brown launched a television ad that faulted his Democratic opponent, Massachusetts Attorney General Martha Coakley, for attacking him.
In the ad, Brown, dressed in a sweater and standing in his kitchen, addressed the camera.
"By now, you've probably seen the negative ads launched by Martha Coakley and her supporters," he said. "Their attack ads are wrong and go too far."
During that campaign, Brown was aided by outside groups that poured millions of dollars into ads attacking Coakley.
This election cycle neither Brown nor Warren can rely on outside groups for harder-edged ads.
This year, the two signed a pledge designed to ban attack ads by outside groups. Under the deal, whichever candidate benefits from a third party ad must write a check for half the value of the ad to a charity named by the other candidate.
It's a sharp contrast from other political contests in the wake of the U.S. Supreme Court's 2010 ruling in the Citizens United case which has made it easier for corporations and wealthy donors to pour millions into campaigns.
Brown has already written two checks for more than $35,000 after outside groups ran ads on his behalf.
Warren's campaign -- taking a cue from Brown's 2010 ad -- recently sent a fundraising appeal accusing Brown of the same kind of attacks he accused Coakley of two years ago.
"He's labeling our supporters ‘insiders, celebrities, elites, occupiers, leftists ,"' Myers wrote in the e-mail. "We can't make Scott Brown stop calling you names, but we can continue to outraise him."
Myers was referring to a Brown fundraising letter in which Brown claims that he's the candidate under fire.
"This is why Washington insiders, celebrities, elites, occupiers and leftists are pouring money into their attack campaign against me," Brown wrote in the fundraising appeal.
For now, the fiery emails appear to be benefiting both campaigns. Each attack by one candidate inevitably inspires a campaign fundraising appeal by the opposing candidate.
The Senate race is on its way to being one of the most expensive in the country and the costliest in Massachusetts history.
Brown's campaign had nearly $15 million in his re-election account as of the end of March compared with the nearly $11 million in Warren's account. Warren, however, was able to raise more than twice as much as Brown in the first three months of the year.