Mayor Bloomberg is taking a tough stance on politicians looking for a middle ground on abortion and said President Bush’s recent Supreme Court appointments “are a powerful reminder of just how important this fight is.”

Mr. Bloomberg yesterday told an audience of abortion rights advocates that “on this issue, you’re either with us or against us,” and he urged them not to let elected officials “nuance themselves” away from a strong commitment to women’s rights.

Mr. Bloomberg’s abortion comments signaled a condemnation of Senator Schumer and other abortion rights advocates who are backing candidates who oppose legalized abortion in this fall’s national elections.

“It could be read as both an implicit criticism and a warning,” a professor of public policy at Baruch College, Douglas Muzzio, said. “This is what he thinks, and he’s speaking his mind regardless of his national ambitions.”

Although Mr. Bloomberg has repeatedly brushed off speculation that he will run for president, he has been actively engaging contentious national issues in the first few months of his second term. Earlier this week, he organized an anti-gun summit with 14 Democratic mayors from across the country, and he yesterday praised a federal court decision that upheld the city’s current use of a nationwide database to trace guns.

Mr. Schumer seemed an obvious target of the mayor’s abortion remarks because the senator recruited an anti-abortion candidate, Bob Casey Jr., to run for the Pennsylvania seat held by Senator Santorum. Mr. Schumer is leading Democrats’ efforts to reclaim the Senate and he is eager to appeal to Pennsylvania voters, who are split on the issue of abortion. Some Democrats believe fielding an anti-abortion candidate is the only way to unseat Mr. Santorum, a leading social conservative.

Calls left for two of Mr. Schumer’s press officers were not returned. A spokesman for Mr. Bloomberg, Stuart Loeser, said the mayor’s comments were not aimed at “anyone in particular.”

In warning that reproductive rights are under threat, Mr. Bloomberg cited President Bush’s appointments of John Roberts and Samuel Alito to the Supreme Court and a recent effort by South Dakota lawmakers to ban abortion. Mr. Bloomberg also voiced support for emergency contraception, noting that women can now obtain the Plan-B drug at any public hospital in the city.

“Reproductive choice is a fundamental human right, and we can never take it for granted,” Mr. Bloomberg said.

The mayor spoke at a fund-raiser for NARAL Pro-Choice New York before a crowd of more than 500 abortion-rights supporters. The audience erupted in applause at the end of his 12-minute speech, when Mr. Bloomberg warned against allowing politicians to find politically expedient solutions to a controversial topic.

Mr. Bloomberg said abortion rights advocates should “exercise vigilance” so elected officials don’t “try to nuance themselves away from that commitment in the interest of political expediency.”

The president of NARAL Pro-Choice New York, Kelly Conlin, said she was “gratified and thrilled” at Mr. Bloomberg’s strong stance in favor of abortion rights. “I think he was issuing a warning not to abandon that position and to be strong,” she said.

The speech came as no surprise to leaders of New York’s Republican and Conservative parties, both of whom oppose abortion. “He’s been very consistent,” the Conservative Party chairman, Michael Long, said. “He was certainly pandering to an audience that likes to hear that kind of rhetoric.”

The GOP chairman, Stephen Minarik, said that while he disagrees with the Republican mayor’s position on abortion, he agrees that “you can’t have it both ways.” He said he respected the mayor for saying what he believes.