NEW YORK, May 1 (UPI) -- A loss in Pennsylvania and a flap over a pastor have bogged down Sen. Barack Obama's quest to be the Democratic U.S. presidential nominee, a poll indicates.

The New York Times (NYSE:NYT)/CBS News Poll released Thursday found Obama in a tighter race with Sen. Hillary Clinton than before. Fifty-one percent of Democratic primary voters polled said they thought Obama would win the nomination -- down from 69 percent a month ago.

Also lower is the percentage of respondents who think the Illinois senator has the best chance of defeating presumptive Republican nominee Sen. John McCain of Arizona. Forty-eight percent said they thought Obama would win in a match-up against McCain -- down from 56 percent a month ago.

Obama had a 10-point loss to Clinton in the April 22 Pennsylvania primary and had to address his controversial ex-pastor, the Rev. Jeremiah Wright. The poll was conducted Friday through Tuesday and may not reflect the controversy's full impact, pollsters said.

The telephone poll was conducted with 1,065 adults and has a sampling error of plus or minus 3 percentage points overall, and 5 percentage points among respondents saying they have voted or will vote in a Democratic primary or caucus.