WASHINGTON — A former House speaker, Newt Gingrich, predicted Democrats will nominate Senator Clinton of New York as their presidential candidate in 2008 and the Republicans have three “very formidable” potential opponents for her.

The top candidates for the Republican nomination, Mr. Gingrich said, are Mayor Giuliani; a former Massachusetts governor, Mitt Romney, and a former senator, Fred Thompson of Tennessee — who hasn’t officially announced his candidacy. He said there was little chance that Senator McCain of Arizona would gain his party’s presidential nod. “These three are very serious people, they’re working very hard,” said Mr. Gingrich, a 10-term U.S. representative from Georgia who led the House between 1994 and 1999, yesterday on “Fox News Sunday” program. The winner of the Republican contest will probably face a Democratic ticket of Mrs. Clinton and Senator Obama of Illinois, he said.

Mr. Gingrich, 64, who has suggested he would consider a presidential campaign, said he won’t “feel any great need to run” if the leading Republicans demonstrate they will be able to run a strong race against Mrs. Clinton. “I think we’ll know that in October,” he said.

Mr. Gingrich dismissed the prospects for Mr. McCain, who started his campaign as the Republican front-runner.

“Senator McCain has taken positions so deeply at odds with his party’s base that I don’t see how he can get the nomination,” Mr. Gingrich said.

Forty-five percent of Democrats surveyed nationally in a Washington Post-ABC News poll earlier this month said they would vote for Mrs. Clinton, compared with 30% for Obama and 12% for a former North Carolina senator, John Edwards.

Mr. Giuliani led Republicans with a 37% backing in a separate Washington Post-ABC News poll, with Messrs. McCain and Thompson receiving 16% and 15%, respectively.

While Mr. Romney trails with 8% support nationally, he leads in recent polls of Republicans in Iowa and New Hampshire, two of the first states to hold nomination contests.

Mr. Thompson, an actor, hasn’t officially declared himself a candidate, though he has set up a political committee to raise money. Mr. McCain’s campaign, meanwhile, has struggled with lagging fund raising and a drop in public opinion polls, coupled with the recent departure of campaign manager Terry Nelson, longtime strategist John Weaver, and several press and advertising aides.

The four-term senator’s support for overhauling immigration laws and his successful drive to modify campaign finance rules has alienated some Republicans and his support of the war in Iraq has cost him the support of some independent voters.