As senators Clinton and Obama crisscross the country seeking the Democratic presidential nomination and sharply criticizing President Bush, they have been collecting hundreds of thousands of dollars from donors who funded one or both of Mr. Bush’s campaigns for the White House.

In the first quarter of this year, more than 150 former Bush donors pitched in for Mrs. Clinton’s campaign, while a similar number anted up for Mr. Obama, according to an analysis of Federal Election Commission data performed for The New York Sun by the Washington-based Center for Responsive Politics.

The motives of those who lend financial support to candidates with divergent views are difficult to assess. For some donors, personal ties to politicians or their top fund-raisers transcend partisan politics. Executives at businesses susceptible to government regulation regularly straddle the field, even supporting multiple candidates in the same race. Some contributors find that their politics change over time or that the politicians they formerly supported failed to follow through on their promises.

On occasion, donors aim to undermine a politician they oppose by supporting a rival they also oppose but view as less of a threat. In 2004, an independent presidential candidate, Ralph Nader, got a wave of large donations from die-hard Republicans who seemed to believe that his presence in the race could siphon support from Mr. Bush’s Democratic opponent, Senator Kerry of Massachusetts.

One donor to Mr. Obama professing sincere disillusionment with Mr. Bush is an investment banker from Chicago, John Canning of Madison Dearborn Partners. “It’s not an isolated trend. It appears to be a significant wave,” he said. “I know lots of my friends in this business are disenchanted and are definitely looking for something different.”

In 2004, Mr. Canning was a Bush Pioneer, meaning he pledged to raise $100,000 for the president’s re-election. However, he told the Sun that his support for Mr. Bush was already fading at that time. “I was probably unenthusiastic, but not as strongly as I am now,” Mr. Canning said. He said he ended up not voting at all. “It wasn’t like I thought Kerry was a good deal.”

Mr. Canning, whose defection to Mr. Obama was reported by Bloomberg News, said he was a big fan of Mr. Bush in 2000. However, he said he later fell out with the president and other Republicans over a dispute involving a brain-injured Florida woman, Terry Schiavo, as well as subjects like global warming, stem cell research and diplomatic relations with Iran and Syria. “A lot of these issues didn’t exist when Bush first ran,” the banker said. “How do you support a guy when he shows the door to everything you believe in?”

Most of those crossing over to support the leading Democrats for 2008 gave to Mr. Bush’s 2000 campaign, though some also backed him financially in 2004. About a half dozen donors to Mr. Bush’s re-election have given to Mrs. Clinton’s presidential campaign. They include the CEO of Yahoo!, Terry Semel; his wife, Jane; a Las Vegas architect, Brad Friedmutter; a San Diego-area developer, Samuel Karp, and an event producer in Washington, Edward Romanoff.

Mr. Karp, a former Israeli paratrooper who described himself as Palestinian, said he is comfortable with both Mr. Bush and Mrs. Clinton because they have “very similar” views on Israel. “I very strongly believe in the U.S. supporting Israel,” he said. “I always call Israel the 51st state.”

Mr. Karp said he is leaning towards Mrs. Clinton now because he considers it scandalous that more than 40 million Americans lack health insurance. “I believe she will do the best to correct this problem,” he said.

Much of the overlap between Mrs. Clinton and Mr. Bush involves people who generally support Democrats but seemed to make an exception to give to the former Texas governor. New Yorkers in this category include the CEO of Warner Music, Edgar Bronfman Jr., as well as Laurence, Renee and Carolyn Belfer, members of a family known for, among other things, their philanthropy to Harvard University.
Mr. Bronfman gave $4,200 to Mrs. Clinton’s presidential bid. He also hosted a widely reported reception for Mr. Obama in February, but campaign finance records do not reflect that he supported the Illinois senator financially in the first quarter.

The somewhat longer list of 2004 Bush donors backing Mr. Obama consists mostly of Illinois residents, but includes several New Yorkers, including a co-founder of Chelsea Piers who is one of Mr. Bush’s Yale classmates, Tom Bernstein of Chelsea Piers, and his wife, Andrea Bernstein of Oxygen Media. Also on that roster are a co-president of Morgan Stanley, Robert Scully, as well as Thomas Pritzker of Chicago, whose family owns the Hyatt hotel chain.

Mr. Bernstein, who is active in human rights causes, was a Bush pioneer and gave $50,000 to the Republican National Committee for the 2004 election. He did not return a call seeking comment for this article.

A Manhattan investment banker who gave $1,000 to Mr. Obama, Steven Greenwald of Credit Suisse First Boston, described his $1,000 gift to Mr. Bush in 2004 as an anomaly. The banker had better recollection of the $2,000 he gave that year to Mr. Bush’s opponent, Senator Kerry of Massachusetts.

“If I gave money to Bush, it’s because one of my buddies asked me to do it. I never voted for the guy. I wouldn’t vote for the guy,” Mr. Greenwald said last week. “My wife is very embarrassed that I did it.”

In a few cases, gifts to Mr. Obama are an abrupt departure from a donor’s history of giving to Republican candidates. A Manhattan investor who hails from a storied banking family, Nathaniel de Rothschild, has given almost exclusively to Republicans in recent years, according to federal records. He gave $2,300 to Mr. Obama in February and the same amount to a former Massachusetts governor seeking the Republican nomination, Mitt Romney. Mr. de Rothschild, who did not respond to a message seeking comment for this article, also gave $5,000 recently to one of Mr. Romney’s political committees.

A former trustee of the conservative Hudson Institute, Edward Wanandi of Lake Forest, Ill., gave $1,000 to Mr. Obama, and gifts of more than twice that size to two Republican presidential contenders, Senator McCain of Arizona and a former Massachusetts governor, Mitt Romney.

Mr. Romanoff, the event staging specialist, gave Mrs. Clinton $4,600 this year after giving Mr. Bush $1,000 in 2000 and $2000 in 2004. The National Republican Congressional Committee paid his firm, PineRock Productions, nearly half a million dollars for support staff and production work in 2004.

Mr. Romanoff did not respond to a request for comment for this article.Giving to more than one candidate in either the Democratic or Republican primary is not uncommon. The Washington Post reported this month that at least 470 donors gave to both Mrs. Clinton and Mr. Obama in the first quarter.

Common names, misspellings, and the use of different addresses and job affiliations by the same donors sometimes make it difficult to determine whether two gifts came from the same person, posing an obstacle in compiling an exact count of donors who backed Mr. Bush and are now backing one of the top Democratic candidates. In addition, donors giving small gifts cannot be tracked because campaigns are not required to itemize donations of less than $200.