WASHINGTON — John Canning has impeccable Republican credentials: He was a Pioneer, one of President Bush’s top fund-raisers. He’s the head of a leveraged-buyout firm. He’s the deputy board chairman of the Federal Reserve Bank of Chicago.

Now, he has given the maximum campaign contribution, $4,600, to a Democratic presidential candidate, Senator Obama.

Mr. Canning says he’s fed up with the Republican Party. “It’s become a party that’s taken Neanderthal positions on things like stem-cell research and global warming,” Mr. Canning, who was appointed to the Fed post in 2004, said in an interview. “I no longer find myself on the same page.”

To Mr. Canning, 62, the party once represented individual rights.

Then in 2005, the Republican-led Congress intervened in an effort to keep Terri Schiavo, a brain-damaged Florida woman, alive against her husband’s wishes. The move was symbolic of Republican positions on social issues that Mr. Canning says he found increasingly frustrating.

Mr. Canning is one of a number of prominent Republicans who have turned against the party. At least two other Bush Pioneers are contributing to Mr. Obama this time, and Mr. Bush’s chief 2004 campaign strategist, Matthew Dowd, assailed the president’s second-term performance in an April 1 New York Times interview.

Mr. Canning said he likes Mr. Obama’s approach to reducing greenhouse gases, his opposition to the Iraq war, and the fact that he’s spent so little time in Washington.

“You know when they say someone’s experienced, if that means they’ve spent a long time in Washington, I don’t know if that’s a pretty good deal,” he said. “He’s the strongest candidate in the entire field from both parties.”

“I’ve probably veered away from the conservative, Republican agenda significantly,” said Mr. Canning, chairman of Chicago-based Madison Dearborn Partners LLC, one of the 20 biggest U.S. private-equity firms.

Because of his role at the Chicago Fed, Mr. Canning said he can’t raise money for Mr. Obama, 45. “He has my emotional support,” he said.

Before co-founding Madison Dearborn in 1992, Mr. Canning spent 24 years at First Chicago Corp. and was executive vice president of First National Bank of Chicago, according to a biography on his firm’s Web site. Madison Dearborn is named after two streets that intersect near the firm’s headquarters.

Mr. Canning also contributed to Republican Mitt Romney’s presidential campaign. “I gave, I think, $1,000 to Mitt Romney as a favor to a friend,” he said. “I don’t have anything to do with him.”

One Bush Pioneer who contributed to Mr. Obama, Framingham, Mass.-based Staples Inc. founder Thomas Stemberg, also gave to Mr. Romney, a former governor of the state. He didn’t return phone calls seeking comment. Another, Chicago publicist Jayne Carr Thompson, declined to discuss her contribution to Mr. Obama except to say, “He is a great representative of Illinois.”

To qualify as a Bush Pioneer, fund-raisers had to bring in at least $100,000 in donations for the 2004 election. Pioneer Ivan Seidenberg, chief executive officer of New York-based Verizon Communications Inc., contributed this time around to Senators Clinton and McCain.

Mr. Obama also is attracting support from Republicans who aren’t prominent in business. It’s hard to go to one of his events and not meet at least one Republican, either curious about the senator or already converted.