Joel Haugen, a Republican fighting a tough congressional race against the Democrats in Oregon, has fallen out with his party. The reason: his surprise endorsement of Barack Obama for the presidency.

"I believe in putting nation before party and my first priority is following my conscience with regard to what is best for America," Haugen said in a statement yesterday. "I have a huge amount of respect for John McCain, but I believe that he has more of a cold war mentality."

Haugen is just one of many Republican politicians, dubbed Obamicans, who have defected to Obama. The latest high-profile desertions include Scott McClellan, President George Bush's former press secretary, who endorses Obama in a CNN programme to be broadcast this weekend, and William Weld, the one-time Republican governor of Massachusetts. Weld, in a statement released yesterday, described Obama as "a once-in-a-lifetime candidate".

Last weekend, Bush's former secretary of state Colin Powell publicly backed the Democratic candidate in Obama's biggest Republican catch so far.

It is unusual to see so many Republican politicians and columnists shift, even allowing for the fact that affiliations are more fluid in the US than Europe.

The Obamicans have a website, republicansforobama.org, listing those who have endorsed Obama. Campaign paraphernalia is also available from Obama's team, including a badge "Republicans for Obama".

According to Haugen's campaign manager, Sarah Tiedemann, he made no secret of his views while fighting for the party nomination, telling the press about his unhappiness with President George Bush's eight years in office and that he was likely to endorse Obama over McCain. He won 70% of the nomination vote.

The Republican party became increasingly "unfriendly", Tiedemann said, and has withdrawn all financial support.

Haugen, as a compromise, is now standing under an independent banner, though he remains a registered Republican. Tiedemann said the reaction among Republicans over his endorsement was mixed. She acknowledged it was "unusual" to make such an endorsement but Haugen "feels the Republican party has got so far from its roots".

Other defectors include Arne Carlson, Republican governor of Minnesota from 1991-1999, who wrote in the Minneapolis Star-Tribune on Thursday: "The choice of Governor Sarah Palin as a running mate, and the resultant shallow campaign based on fear and suspicion, looks frighteningly similar to the politics of Karl Rove [Bush's campaign strategist]." He described Obama as having "the potential to become a truly great president".

McClellan. who published a book this year critical of the Bush administration, accusing it of lying, told CNN he would vote for Obama because he offered the best chance of changing the way Washington works.