A last-ditch American effort to head off a Palestinian bid for membership in the United Nations faltered. President Obama tried to qualify his own call, just a year ago, for a Palestinian state. And President Nicolas Sarkozy of France stepped forcefully into the void, with a proposal that pointedly repudiated Mr. Obama’s approach.

The extraordinary tableau Wednesday at the United Nations underscored a stark new reality: the United States is facing the prospect of having to share, or even cede, its decades-long role as the architect of Middle East peacemaking.

Even before Mr. Obama walked up to the General Assembly podium to make his difficult address, where he declared that “Peace will not come through statements and resolutions at the U.N.,” American officials acknowledged that their various last-minute attempts to jump-start Israeli-Palestinian negotiations with help from European allies and Russia had collapsed.

American diplomats turned their attention to how to navigate a new era in which questions of Palestinian statehood are squarely on the global diplomatic agenda. There used to be three relevant players in any Middle East peace effort: the Palestinians, Israel and the United States. But expansions of settlements in the West Bank and a hardening of Israeli attitudes have isolated Israel and its main backer, the United States. Dissension among Palestinian factions has undermined the prospect for a new accord as well.

Finally, Washington politics has limited Mr. Obama’s ability to try to break the logjam if that means appearing to distance himself from Israel. Republicans have mounted a challenge to lure away Jewish voters who supported Democrats in the past, after some Jewish leaders sharply criticized Mr. Obama for trying to push Israel too hard.

The result has been two and a half years of stagnation on the Middle East peace front that has left Arabs — and many world leaders — frustrated, and ready to try an alternative to the American-centric approach that has prevailed since the 1970s.

“The U.S. cannot lead on an issue that it is so boxed in on by its domestic politics,” said Daniel Levy, a former Israeli peace negotiator in the government of Ehud Barak. “And therefore, with the region in such rapid upheaval and the two-state solution dying, as long as the U.S. is paralyzed, others are going to have to step up.”

Mr. Obama himself seemed to forecast this back in May when, speaking to the American Israel Public Affairs Committee, he warned that events in the Middle East could lead to a challenge to the status quo if the Israelis and Palestinians did not move quickly toward a peace deal.

“There’s a reason why the Palestinians are pursuing their interests at the United Nations,” Mr. Obama said then. “They recognize that there is an impatience with the peace process, or the absence of one, not just in the Arab world, in Latin America, in Asia, and in Europe. And that impatience is growing, and it’s already manifesting itself in capitals around the world.”

The Palestinian president, Mahmoud Abbas, delivered on the threat. He announced last Friday his plans to go to the Security Council in a quest for Palestinian membership in the United Nations and international legal recognition of statehood, putting Mr. Obama in the position of having to stand in the way. Israel and its allies in Congress, where Prime Minister Benjamin Netanyahu of Israel enjoys broad influence, were sharply opposed.

So on Wednesday, Mr. Obama “did exactly what he had to do,” said David Rothkopf, a former Clinton administration official and a visiting fellow at the Carnegie Endowment for International Peace. “He made a clear statement for what is a clear U.S. position and put himself squarely as a champion of the status quo.”

Mr. Netanyahu, Mr. Rothkopf said, “has managed to read the U.S. political situation perfectly, making Obama acutely aware that he could be losing part of his base, and that, I think, in turn is what has locked Obama in.”

The Palestinians have never fully trusted the United States to serve as an honest broker with Israel. But its credibility with the Palestinians has crumbled with the recognition that Mr. Obama may not have the clout to press the Israelis into a peace deal that requires significant compromises.

“The president in his speech at the U.N. today admitted that the U.S. somehow failed in bridging the gap between the two sides,” the Palestinian representative in Washington, Maen Rashid Areikat, said in an interview on Wednesday. “He said that he feels frustration and he understands the frustration of everybody. That’s good, but I think it goes much deeper. I think what the U.S. administration needs to say is why it failed.”