One of the biggest advantages Senator Clinton enjoys as she launches her presidential bid is the vast web of politically active Democrats who worked in the federal government under her husband, President Clinton. But not everyone who served during the Clinton years is promoting a reprise.

A handful of top Clinton administration officials and a smattering of lower-ranking ones have taken up with Mrs. Clinton’s rivals for the Democratic nomination. Most cite pre-existing personal or professional loyalties. In some instances, however, the Democratic activists seem to have concluded that they will have more of an impact in the leaner ranks of a rival campaign than in the hierarchy of Mrs. Clinton’s long-planned bid. And a former White House counsel to Mr. Clinton, Abner Mikva, told The New York Sun he thought the candidate he is backing, Senator Obama, has a better chance of winning than Mrs. Clinton does. The highest-ranking defectors include a political aide who shadowed Mr. Clinton for years and is advising Senator Dodd of Connecticut, Douglas Sosnik; a former secretary of commerce now backing Senator Obama of Illinois, William Daley, and a former chief of staff to Vice President Gore now in the camp of Senator Biden of Delaware, Ronald Klain.

“Most of the people have some sort of pre-existing connection or relationship,” a former Clinton White House aide, Christopher Lehane, said. He said he doubted there were hard feelings in those circumstances, though there might be in cases where people had no obvious prior ties to one of Mrs. Clinton’s rivals. “There are different gradations to all of this,” Mr. Lehane added.

A former White House lawyer and longtime friend of the Clintons, Lanny Davis, said he knew of no one who worked on Mrs. Clinton’s staff and was now backing another candidate. “I don’t know of any defections,” he said. “That says a lot about her as a human being.” One of the challenges Mrs. Clinton faces in trying to maintain the loyalty of mid-level operatives is that so much of her campaign operation has been lined up for years. “That bus is full,” one Democratic consultant, Joseph Trippi, told The New York Sun recently.

One former top adviser to Mr. Clinton said more painful cleavages would have taken place if Vice President Gore was competing with Mrs. Clinton for the nomination. “If it were a Gore-Clinton race, these decisions would have been in higher relief. It would have been a messier situation,” the ex-aide, who asked not to be named, said.

A spokesman for Mrs. Clinton, Howard Wolfson, said yesterday that the senator is more than satisfied with the backing of former government officials. “We are very pleased with the overwhelming support and encouragement Senator Clinton has gotten from administration alums,” he said.

Asked about the defectors, Mr. Wolfson said, “All good folks and we wish them well.”

The recent entry of Mr. Obama into the presidential mix had the immediate effect of dislodging one prominent Democratic leader in Congress from the Clinton camp. In April, Rep. Rahm Emanuel of Illinois indicated he was solidly behind Mrs. Clinton. “I’m supporting Hillary Clinton. I’m public about it,” he told Bill Maher on HBO’s “Real Time.”

However, Mr. Emanuel, who was a key political and strategic staffer in the Clinton White House, is now describing himself as uncommitted. He has painted the choice between Mrs. Clinton and Mr. Obama as a painful one.

“I’m hiding under the desk,” Rep. Emanuel, the new chairman of the House Democratic Caucus, told the Chicago Tribune in a story posted on the Web yesterday. “I’m very far under the desk, and I’m bringing my paper and my phone….At some point, if I have to make a decision, I will.”

Rep. Emanuel could also face some family pressure. His brother, Ariel, who is a prominent talent agent in Los Angeles, is enthusiastically supporting Mr. Obama. The well-connected agent has already set up one California event for the Illinois senator and is planning another in February.

Mr. Klain said his decision to support Mr. Biden was the result of “a prior debt of loyalty” stemming from his work as a Senate staffer beginning in the 1980s.

“I am a big Hillary Clinton fan, and if Senator Biden chooses not to run, I will happily support her,” Mr. Klain said in an e-mail to the Sun yesterday. “But I would have never met Mrs. Clinton, if Joe Biden hadn’t called Bill Clinton and recommended me for a job in the administration.”

Mr. Sosnik and Mr. Daley did not return calls about their situations. Friends of Mr. Sosnik noted that he has been close to Mr. Dodd since serving as a driver on Mr. Dodd’s first Senate campaign in 1980.

Mr. Daley’s backing of Mr. Obama is generally seen as driven by geographic factors as well as political connections between Mr. Obama and Mayor Daley of Chicago, who is the former commerce secretary’s brother.

A political science professor at the University of Illinois, Dick Simpson, said the Daley family’s support for Mr. Obama, who is partly of African descent, is due in part to the fact that the mayor is facing two African-American challengers in his re-election bid this fall. “By supporting a black candidate for president, he can win some of Barack’s support in the African-American community,” Mr. Simpson said. “As Tip O’Neill used to say, ‘All politics is local,’ and in Chicago, it’s very local.”

Other ex-Clintonites in the Obama camp include a former Treasury Department official who went to law school with the Illinois senator, Michael Froman; a former White House Congressional liaison, Broderick Johnson; a former Treasury aide, Karen Kornbluh, and a former White House health policy aide who will handle research for Mr. Obama’s bid, Devorah Adler. Mr. Obama has also won the allegiances of a former national security adviser to Mr. Clinton, Anthony Lake; a former White House adviser on Africa policy, Susan Rice, and a former White House counsel, Abner Mikva.

“I knew Obama before I went to the White House,” Mr. Mikva told the Sun last night. “I have a high regard for Hillary. I think Barack will be a better candidate and has a better chance of winning.”

A former senator from North Carolina who was a presidential candidate in 2004 before becoming the Democrats’ vice presidential nominee that year, John Edwards, also has a substantial cadre of ex-Clinton staffers for 2008, many of whom worked for him the last time around. They include an advertising consultant and former Clinton speechwriter, Jonathan Prince; a White House spokeswoman, Jennifer Palmieri; a National Security Council staffer, Miles Lackey, and a White House education policy aide, James Kvaal.

As Mr. Biden navigates the shoals of Iraq policy, his top staffer on such issues is another former NSC staffer, Antony Blinken. A former Iowa governor running for the White House, Thomas Vilsack, also has some former Clinton talent on his staff, including an Education Department veteran, Cheryl Parker Rose.

The ex-Clinton aide mounting the most direct challenge to Mrs. Clinton may be a man who served as energy secretary in the Clinton administration, Governor Richardson of New Mexico. He is considering entering the Democratic presidential race himself.

“The general pattern is the Clintonites have held,” Mr. Davis said. “If James Carville came out for Obama, I’d be surprised.”

[Senator Obama may not have officially said he is running for president, but already his constituents want to know: Does he have enough experience to occupy the White House?

“My expectation would be if I ran, I will run through the gauntlet, and people will ask me tough questions just like you just did,” Mr. Obama replied. “And hopefully, the American people will decide whether or not I’ve got the experience.”

Mr. Obama, a Democrat of Illinois, took questions yesterday from a group of about 100 visiting constituents who were eager to hear how he might perform if he were to win the presidency.

“There are reporters everywhere here,” Mr. Obama joked. “I appreciate you doing their dirty work.”

During the session, there was so much interest in Mr. Obama’s presidential ambitions that it was difficult for Illinois’s senior senator, Senator Durbin, also a Democrat, to draw much attention.

Mr. Obama, 45, formed a presidential exploratory committee on Tuesday, and plans to make an official announcement February 10. He assured the audience that any presidential campaign would not detract from his Senate voting record or his work on their behalf. “I think what you try to do is to manage to make those votes that are going to make a difference and you are going to miss votes eventually that where your vote is not decisive,” he said.

Mr. Obama also gave his quick view of Social Security, Medicare, and Medicaid.

“The crisis in Social Security, I believe, is overstated in the sense that, if we make some adjustments to the system, as we did in 1983, we can assure the solvency of the system,” Mr. Obama said.]