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Teresa Sayward, a Republican New York Assembly member who was first elected in 2002, said she will vote for Mr. Obama because Mr. Romney — and all of the other Republican presidential candidates this year — proposed what she considers anti-women policies.

“It’s disheartening for me to see our party move away from what it was always about, and that is to stay out of people’s lives, let them live their lives, don’t impose their religion on anybody else, don’t impose their feelings, let them live and uphold the Constitution,” she told a New York political TV program in March.

“I really, truly think that the candidates that are out there today for the Republican side [for president] would take women back decades.”

In the 2008 presidential election, about 9 percent of self-identified Republicans who voted supported Mr. Obama, exit polling showed. The Republican candidate, Sen. John McCain of Arizona, received about 10 percent of Democratic voters.

It’s unlikely that the president will duplicate his crossover rate of four years ago, polling and political analysts suggest. Gallup polling shows that about 6 percent of Republicans say they will vote for the president, and the same percentage of Democrats say they will support Mr. Romney. A Pew Research survey taken in July showed identical numbers.

Gone is the advantage Mr. Obama had four years ago, when he won the support of some moderate Republicans who opposed key Bush-era policies such as the Iraq War, the USA Patriot Act and the administration’s penchant for spending.

Voters viewed both major 2008 presidential candidates as centrists and consensus-building candidates — at least compared with the highly partisan climate at the time, said national pollster John Zogby. This year, voters are much more likely to cast ballots for their respective party’s nominee.

Enthusiasm among Republicans also is much higher than in 2008, and low approval ratings for Mr. Obama aren’t helping him attract Republicans.

“It’s [political] war, essentially,” Mr. Zogby said. “Even when voters have expressed dismay and have been disappointed in either candidate, they go to the ‘not sure’ column as opposed to saying they’re going to vote for the other guy.”

Mr. Martin, who heads the Republicans for Obama group and who voted for the Illinois Democrat in 2008, said he has taken much flak from other members of his party, who have called him communist, socialist and liberal. But he said a big part of his mission is to remind Republicans of their party’s history of being a coalition that accepts divergent views.

“People don’t realize that 4 million Republicans voted for Obama in 2008, and moderate Republicans — those who aren’t in the hard right and most vocal parts of the party — a lot of times people forgot about us,” he said.

Whether he stays with the Republican Party depends on what happens in November. An Obama win, he hopes, would foster discussion and self-examination within the GOP to return to its “big tent” roots.

He said he fears that a Romney win would serve as justification for conservatives to continue remaking the party in their likeness.

“I still have a lot of hope that [the GOP] can become more accepting toward moderate Republicans, which in many ways I am, and I want to help reform the Republican Party. I’m not ready to leave,” Mr. Martin said.

“I hope this is the last time I support a Democrat, but I’ll just have to see how the Republican Party responds to this election first.”