Mayor Giuliani, who is favored among Republican presidential candidates, also leads the top Democratic contenders in head-to-head match-ups, a poll found.

Mr. Giuliani leads Senator Clinton of New York by 49% to 40% and is favored over Senator Obama of Illinois 44% to 41%, the Quinnipiac University poll showed. Mr. Giuliani leads former Vice President Gore, who isn’t running in the 2008 race, by seven points, 48% to 41%.

Among nearly a dozen possible Republican candidates, Mr. Giuliani leads the field, even as his share of support among party voters fell to 27% from 40% in a February poll. Senator McCain of Arizona received the support of 19% of Republicans, while a former senator of Tennessee, Fred Thompson, who hasn’t said whether he would seek the Republican nomination, received 14%.

A former Massachusetts governor, Mitt Romney, and a former House speaker, Newt Gingrich, each received 8% of the support, the poll found.

Those who responded to the survey said they favored voting for a Democrat rather than a Republican in the 2008 election by a margin of 43% to 34% with no candidates named.

Among eight Democratic candidates, Mrs. Clinton was favored with 32% of party voters backing her, down from 38% in a February survey.

Mr. Obama was backed by 18% of Democratic respondents, and a former North Carolina senator, John Edwards, had the support of 12%. Fifty-one percent of those surveyed said Congress should set a timetable for withdrawing all U.S. troops from Iraq.

President Bush’s approval rating was at 35%, similar to the February survey. Thirty-one percent said they approve of the way Mr. Bush is handling the war in Iraq.

The poll conducted by Quinnipiac University, based in Hamden, Conn., surveyed 1,166 American voters from April 25 to May 1. It had a margin of error of plus or minus 2.9 percentage points. The margin for error for the 469 Republicans among those polled was plus or minus 4.5 percentage points. The margin was 4.4 percentage points for the 499 Democrats polled.