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HANOVER, N.H. — Hoping to build on the momentum that has led to his rise in the polls in Iowa and New Hampshire, Senator Bernie Sanders on Thursday evening announced the endorsement of Paul G. Kirk Jr., a confidante of Senator Edward M. Kennedy and a former Democratic National Committee chairman.

“Let me again thank Senator Kirk for his very generous support,” Mr. Sanders said at a news conference at Dartmouth College before a campaign rally, calling the endorsement “another indication of the kind of momentum our campaign has.”

As Mr. Sanders, in a blue-stripe shirt and holding a yellow legal pad, stood to his right, Mr. Kirk took to the lecturn and expressed Sanders-like impatience with the public theater surrounding the endorsement.

“I feel a little bit like jack-in-the-box,” Mr. Kirk, wearing a starched white shirt and purple tie, said dourly. “But I’m delighted to be here.”

In his remarks, Mr. Kirk said that of all the presidential candidates, only Mr. Sanders was telling the truth that the most “pernicious internal peril” to American democracy was money flooding the political system.

As a small room of reporters in a faculty lounge perused Mr. Kirk’s five-page statement explaining his endorsement (“a second closely connected and equally uncomfortable truth…”) Mr. Kirk said that Mr. Sanders, in apparent contrast to Hillary Clinton, had “genuine empathy” and the ability to drum up the enthusiasm of young voters looking to change the political system.

Calling Mr. Sanders a “forthright son of Brooklyn,” Mr. Kirk said that he was “here tonight to join in this untiring effort.”

The Sanders campaign treated the endorsement of Mr. Kirk, who was appointed to briefly fill the Massachusetts Senate seat left vacant by Mr. Kennedy’s death in 2009, as a major event. But the support of Mr. Kirk, who spoke in his remarks about President John F. Kennedy, is but an echo of the shot heard round the political world when Senator Kennedy and his niece Caroline endorsed Barack Obama against Hillary Clinton in the early stages of the 2008 Democratic primary.

At the news conference, Mr. Sanders, who has been engaging in intensifying exchanges with Mrs. Clinton over issues including health care, gun control and electability, was asked about the intensifying criticism from Mrs. Clinton.

In response to one question, Mr. Sanders promised that he would release information about how he would pay for his single-payer health care plan before the Iowa caucus on Feb. 1. The Clinton campaign has accused him of backtracking on that vow.

Mr. Sanders also responded to anger expressed by Clinton campaign aides Thursday who accused him of retreating on a promise not to run negative ads by releasing one denigrating Mrs. Clinton’s vision for overhauling Wall Street.

“Today they were mad at me, yesterday they were mad at me, tomorrow they will be mad,” Mr. Sanders said, before arguing “that was not a negative ad” and complaining that Mrs. Clinton, in effect, had started it, with an ad that tacitly questioned his commitment to gun control.

“You tell me what that ad was about,” he said.

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