UNITED NATIONS — The Senate Foreign Relations Committee will hold hearings Thursday on the renomination of John Bolton as ambassador to the United Nations.

The hearing was made possible yesterday after the main holdout in last year’s bruising nomination fight, Senator Voinovich, a Republican of Ohio, announced his intention to support Mr. Bolton’s nomination.

After a Senate impasse last year that resulted in a vacant Turtle Bay ambassador’s seat, President Bush named Mr. Bolton ambassador in a recess appointment last summer. The ambassador’s term, which began August 1, 2005, ends December 31.

The president could reappoint Mr. Bolton during the congressional recess, but that would entail no funding, and the ambassador would have to work without a salary. If it does not find a new candidate to replace Mr. Bolton, the administration must send him for reconfirmation.

Next week’s hearing at the 18-member Senate committee will begin the process. If the committee recommends it, Mr. Bolton’s nomination will be presented for a vote on the Senate floor.Last year, the committee sent the nomination to the floor without such a recommendation, after a 10–8 vote that split along party lines.

The administration decided to bypass the rest of the lengthy process and appointed Mr. Bolton without congressional support.

Mr. Voinovich was the lone Republican on the committee who said he would oppose Mr. Bolton’s appointment. “Should the president send his renomination to the Senate I will vote to confirm him,” Mr. Voinovich wrote in the Washington Post’s op-ed page yesterday. “And I call on my Democratic colleagues to keep in mind the current situation in the Middle East and the rest of the world should the Senate have an opportunity to vote.”

Mr.Voinovich, who is not up for re-election in this fall’s election, added, “For the good of our country, the United Nations and the free world, we must end any ambiguity about whether John Bolton speaks for the United States so that he can work to support our interests at the United Nations during this critical time.”

With Mr. Voinovich’s about-face, last year’s balance of power has now tipped. But even some of Mr. Bolton’s most adamant supporters are not sure the renomination, which will allow Mr. Bolton to serve at the United Nations at least until the end of the Bush presidency, will sail through as the political season heats up. Democrats yesterday signaled they would fight hard.

“Mr. Bolton did not get a vote last year because the Administration refused, with no justification, to allow the Senate to review documents highly relevant to his nomination,” Senator Biden, a Democrat of Delaware who led last year’s fight against the nomination, said yesterday in a statement.

His reservations have intensified since then, Mr. Biden added. “Mr. Bolton’s performance at the U.N. also confirms my conviction that he is the wrong person for this job.” Indicating that the nomination will be hardfought, Mr. Biden said, “Instead of wasting time and playing politics, the Administration should nominate someone else to take Mr. Bolton’s place when his recess appointment expires this fall.”