It seemed next to impossible just a few days ago: the idea of Senator Obama competing against — much less beating — Senator Clinton on her home turf in next month’s New York State Democratic primary. But in the wake of his victory in Iowa and polls showing him pulling ahead of Mrs. Clinton in New Hampshire, site of today’s primary, the idea of Mr. Obama winning New York has entered the realm of plausibility.

“I am worried,” Mayor Koch, who backs Mrs. Clinton, said. “The whole question is whether the country is caught up in an Obama fever. I still believe the best will prevail and she is the best.”

A state senator who is one of the few New York Democratic elected officials who supports Mr. Obama, Bill Perkins, said the calculus for New York delegates has changed. “Assuming all the polls bear out like they did in Iowa, then we are going to have a race,” Mr. Perkins, who represents Harlem, said.

“Pre-Iowa, Hillary Clinton was considered the inevitable and I think obviously, now having lost Iowa and the polls showing the likelihood of a significant defeat in New Hampshire, that does not apply to her or to anyone else for that matter,” he said.

A December 17 poll by Quinnipiac University showed Mrs. Clinton leading Mr. Obama by 55% to 17% among likely Democratic primary voters in New York. No new state polls have been released since last week’s Iowa caucus.

With a gale of momentum at his back since his strong margin of victory in Iowa and a USA Today/ Gallup poll released yesterday showing Mr. Obama with 41% support in New Hampshire versus Mrs. Clinton’s 28%, everything, including New York State, seems to be in play.

Tomorrow, Mr. Obama will be holding a fund-raiser at New York City’s Grand Hyatt, and Mr. Perkins said more volunteers would be out in subway stations in the coming days as part of a voter registration drive prior to the January 11 deadline.

Of New York State’s 281 delegates, 151 will be elected proportionally based on the ballot results of the February 5 primary. Forty-five of the remaining 130 will be chosen by party leaders, and the remaining 85 will be selected at a New York State Democratic Committee meeting in May.

While the committee is prohibited from endorsing candidates before the primaries, the chairwoman of the New York State Democratic Party, June O’Neill, and her co-chairman, David Pollack, have personally endorsed Ms. Clinton.

“It was never going to be a sweep in New York because you can’t do it in the Democratic primary,” Quinnipiac pollster Mickey Carroll said.

“It was certain, even when Hillary Clinton was riding unchallenged, that Obama would get some votes in Brooklyn, maybe in Manhattan, and now there is something to be said for momentum or emotion,” he said.

Political consultant Hank Sheinkopf said much will hinge on New York’s African-American voters. If Mr. Obama were to achieve a string of victories in New Hampshire and then in South Carolina, African-American voters “will see that he is the real deal, there is no question about it and they are going to be very pumped up about Barack Obama, as they should be,” he said.