Senator Edward M. Kennedy was set to endorse Senator Barack Obama for president on Monday as part of an effort to lend Kennedy charisma and connections to Mr. Obama as he heads into a 22-state showdown with Senator Hillary Rodham Clinton.
The disclosure of the endorsement, which followed a public appeal on Mr. Obama's behalf by Caroline Kennedy, the daughter of President John F. Kennedy, was a blow to the Clinton campaign and pits leading members of the nation's most prominent Democratic families against one another.
Beginning with a joint appearance and rally in Washington on Monday, Mr. Kennedy, a major figure in party politics for more than 40 years, intends to campaign aggressively for Mr. Obama, heading West this week, followed by appearances in the Northeast. Strategists see him bolstering Mr. Obama's credibility for the office and providing particular benefits with union members and Hispanics, as well as the party base.
Both the Clintons and their allies had pressed Mr. Kennedy for weeks to remain neutral, but Mr. Kennedy had become increasingly disenchanted with the tone of the Clinton campaign. He and former President Bill Clinton had a heated telephone exchange earlier this month over what Mr. Kennedy considered misleading statements by Mr. Clinton about Mr. Obama, as well as his injection of race into the campaign.
Mr. Kennedy called Mr. Clinton on Sunday to tell him of his decision.
The endorsement appears to support assertions that Mr. Clinton's campaigning on behalf of his wife in South Carolina has in some ways hurt her candidacy.
Campaign officials, without acknowledging any faults on Mr. Clinton's part, have said they will change tactics and try to shift Mr. Clinton back into the role he played before her loss in the Iowa caucuses, emphasizing her record and experience and steering clear of criticizing Mr. Obama.
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