President Barack Obama sought to rally Massachusetts Democrats on Sunday, telling a large crowd at Northeastern University in Boston that they have a “unique and special responsibility— to fill the late-Sen. Edward Kennedy’s (D) seat in Tuesday’s special election.And the best person to fill that seat, he said, is Massachusetts Attorney General Martha Coakley (D).“Martha knows the struggles Massachusetts working families face,— said Obama, pointing to her humble roots in North Adams, Mass., and saying she “has a track record— of putting people first.And Obama sought to tear down state Sen. Scott Brown’s (R) claims that he is the independent-minded outsider in the race. “It’s easy to say you’re independent and you’re going to bring people together and all that stuff, until you actually have to do it,— he said.Coakley, Obama said, has “the character to be independent,— while Brown, whom Obama referred to only as “Martha’s opponent,— “is already walking in lock step with Washington Republicans.—After resisting involvement in the race, the president swept into Boston on Sunday afternoon to help bolster a badly shaken Coakley campaign, which started out the general election contest the heavy favorite but has sunk in the polls amid a series of gaffes and a surprisingly strong challenge from Brown’s energetic underdog campaign.A win by Brown in Massachusetts would endanger the Democrats’ ability to pass health care through the Senate, not to mention complicate the rest of their legislative agenda. Brown has already promised he would become Republicans’ 41st vote against the health care bill, enabling a GOP filibuster.Obama, who was interrupted briefly during his speech by the shouts of anti-abortion protesters, told the crowd he needed Coakley as a partner in Washington.“We need you!— he shouted, as he wrapped up his speech.The state’s Democratic establishment was out in full force for the event, with Sen. John Kerry (D-Mass.) and Kennedy’s widow, Vicki, warming up the crowd.The Boston Globe reported that thousands of people lined up outside Northeastern University early Sunday to get into the event, a much-needed jolt of enthusiasm for the Coakley campaign, which has struggled to counter the grass-roots energy of Brown supporters.Brown held his own concurrent “People’s Rally— in Worcester, Mass., featuring “hometown heroes— Curt Schilling, the former Red Sox pitcher Coakley mistakenly dubbed a “Yankees fan— in a Friday interview; former Boston College quarterback Doug Flutie; and the actor John Ratzenberger, a.k.a. Cliff from Boston-centered television series “Cheers.—Republicans have been working over the weekend to underscore the contrast between Obama’s “politics of hope,— which he proffered in the 2008 presidential campaign, and the increasingly negative rhetoric coming from the Coakley campaign.Both campaigns, in fact, have been ramping up the tone and intensity of the attacks, though Democrats in particular seem to be trying to throw everything but the kitchen sink at Brown.Over the weekend, they’ve gone after Brown for everything from questioning whether Obama’s parents were married when he was born in a 2008 interview to not providing health care to his campaign staff.The Brown campaign filed a complaint against the Massachusetts Democratic Party on Saturday over a mailing that accuses Brown of wanting to allow hospitals to turn away all rape victims. The Boston Globe called the claim “a major misrepresentation.—