BIRMINGHAM, Ala. — Senator Edward M. Kennedy intends to endorse the presidential candidacy of Senator Barack Obama during a rally on Monday in Washington, associates to both men confirmed, a decision that squarely pits one American political dynasty against another.

The expected endorsement, coming after Mr. Obama’s commanding victory over Senator Hillary Rodham Clinton in the South Carolina Democratic primary on Saturday, may give Mr. Obama further momentum in his campaign for the nomination.

As Mr. Obama flew here on Sunday, he smiled when asked by reporters about Mr. Kennedy’s plans, saying: “I’ve had ongoing conversations with Ted since I’ve got into this race.” He learned of Mr. Kennedy’s decision through a telephone call on Thursday, aides said, three days before the South Carolina primary.

Of all the endorsements in the Democratic Party, Mr. Kennedy’s is viewed as among the most influential. The Massachusetts senator had vowed to stay out of the presidential nominating fight, but as the contest expanded into a state-by-state fight — and given the tone of the race in the last week — associates said he was moved to announce his support for Mr. Obama.

Mrs. Clinton said Sunday that she had not expected to get the endorsement, yet aides conceded that they hoped Mr. Kennedy would remain neutral.

The endorsement will be announced at a rally at American University on Monday, hours before the State of the Union Address at the Capitol.

One day after defeating Mrs. Clinton in South Carolina by 28 points, Mr. Obama flew to Georgia to speak at a church service before traveling on to Alabama for a rally. Both states are among those holding primaries in the next stop in the Democratic presidential nominating fight on Feb. 5.

Mr. Kennedy, the latest in a string of senators to get behind Mr. Obama, is said by associates to be drawn to the Illinois senator because of his ability to motivate a new generation of Democrats. His niece, Caroline Kennedy, made a similar argument in an Op-Ed piece in The New York Times on Sunday.

“For somebody who, I think, has been such an important part of our national imagination and who generally shies away from involvement in day-to-day politics to step out like that is something that I’m very grateful for,” Mr. Obama told reporters on Sunday, referring to Ms. Kennedy.

During Mr. Obama’s three years in the Senate, he has worked to build allies and gain friendships with many of his colleagues. While Mr. Kennedy and Mr. Obama have not been particularly close, Mr. Obama quickly gained the admiration of the Kennedy family.

Besides providing Mr. Obama with an important boost for his campaign, his strong victory in South Carolina also raised questions about the Clinton camp’s strategy of aggressive attacks on him. But in an interview on Sunday on the CBS News program “Face the Nation,” Mrs. Clinton said she would not back off from taking shots at her chief rival’s positions, saying, “It’s important we draw these contrasts.”

“The idea that somehow someone’s record, someone’s words are off-limits, I’ve never seen that in American politics,” Senator Clinton added.

With each of the top Democrats having notched two primary victories, it appears increasingly likely that the party’s presidential nominating fight could extend well beyond the multistate primary elections on Feb. 5.

John Edwards, the former North Carolina senator, received 18 percent of the vote in South Carolina. Despite the disappointing finish by Mr. Edwards, who was born in South Carolina, the former senator insisted that he will remain in the race. Mr. Edwards’s delegates could eventually play a key role in the fight between Mr. Obama and Mrs. Clinton.

The next big contest comes on Tuesday in Florida. In the Republican race, Senator John McCain received the endorsement of the state’s popular Republican governor, Charlie Crist. On Sunday, Mr. McCain again assailed the Iraq policies of a top rival, Mitt Romney, castigating the former Massachusetts governor for having suggested earlier some sort of unannounced timetable for withdrawal.

“Governor Romney obviously said there have to be ‘timetables,” although they had to be secret,” McCain said on “Meet the Press” on NBC. “If we had done that, as the Democrats and some Republicans wanted to do, we would have lost that surge and al-Qaeda would be celebrating a victory over the United States of America.”

Mr. Romney has demanded an apology from Mr. McCain, saying the Arizona senator had been dishonest in his description.

The Florida primary could be critical for the Republican campaign of Rudolph W. Giuliani, the former New York mayor. Mr. Giuliani on Sunday predicted victory in Florida, seeing the large number of early absentee votes cast so far as a helpful sign and saying his tax-cutting promises were resonating with voters. As of Friday night, nearly 400,000 party Republicans had cast early votes, compared with the 200,000 who cast votes at this point in 2006. Florida is one of 37 states to permit early voting, and Mr. Giuliani has made an effort to get his supporters to vote early over the past month.

Democratic candidates are now taking their campaigns to states like California and New York for contests that hold vast numbers of delegates. Tuesday’s primary in Florida will play an unusual role in this election because the Democratic National Committee has said it would not seat Florida’s delegates because the state is holding the primary earlier than party rules apply.

Democratic candidates have not actively campaigned in Florida because of the party’s decision, but that does not mean a race has not been conducted by Florida supporters of Mrs. Clinton and Mr. Obama. As of Friday night, nearly 350,000 Democrats had cast early votes — exceeding the turnout in Iowa, New Hampshire and Nevada — and party officials predicted that roughly 400,000 will have voted by election day.

Jeff Zeleny reported from Birmingham, Ala., and Brian Knowlton from Washington. Adam Nagourney contributed reporting from Boca Raton, Fla.