WASHINGTON, May 24 - The Ohio Republican whose opposition to John R. Bolton nearly stalled his nomination in committee circulated a letter on Tuesday urging colleagues to vote against Mr. Bolton when his name reaches the Senate floor, possibly this week.
The renewed opposition from the senator, George V. Voinovich, was addressed to all his colleagues, but it was aimed particularly at fellow Republicans in a chamber in which the party holds a 55-to-44 majority. At least five Republicans would have to join Mr. Voinovich in opposing Mr. Bolton's nomination as United Nations ambassador in order to defeat it.
In the letter, Mr. Voinovich said that while he had been "hesitant to push my views on my colleagues" during his six years in the Senate, he felt "compelled to share my deep concerns" about the nomination.
"In these dangerous times, we cannot afford to put at risk our nation's ability to successfully wage and win the war on terror with a controversial and ineffective ambassador to the United Nations," Mr. Voinovich wrote. He urged colleagues to "put aside our partisan agenda and let our consciences and our shared commitment to our nation's best interests guide us."
The White House remains strongly in favor of Mr. Bolton's nomination, and it is unusual for a Republican to break ranks so publicly with President Bush.
"We are confident John Bolton will be confirmed," Erin Healey, a spokeswoman for the White House, said Tuesday. "Many highly respected people who know him well support his nomination, and we urge the Senate to act quickly."
In addition to sending out his letter, Mr. Voinovich was also making telephone calls and meeting with other Republican senators to urge them to oppose Mr. Bolton's nomination, according to two Senate Republican officials. A copy of his letter, dated May 23 but not circulated until Tuesday, was provided by a Senate Democratic aide opposed to the nomination.
The Senate's Republican leaders on Tuesday were trying to win agreement from Democrats on a plan that could allow a vote on Mr. Bolton by the end of the week. Senate Democrats have strongly opposed the nomination, and the party's leaders in the Senate were weighing possible moves to defeat it or procedural moves to delay or prevent a vote.
It is not clear whether any Republicans might join Mr. Voinovich in breaking ranks with the White House.
Among the 10 Republicans on the Senate committee, three joined Mr. Voinovich in expressing reservations about Mr. Bolton's nomination. The Republicans on the panel agreed only to send the nomination to the full Senate without an endorsement, an unusual move.
On Tuesday, however, spokesmen for two of those Republicans, Senators Chuck Hagel of Nebraska and Lincoln Chafee of Rhode Island, said their bosses expected to vote in favor of Mr. Bolton when his name comes before the full Senate. A spokeswoman for the third, Senator Lisa Murkowski of Alaska, said the senator had told reporters from her home state that she was "likely to support Bolton's nomination on the floor."
A spokesman for another Republican, John Thune of South Dakota, said Tuesday afternoon that Mr. Thune "hasn't made any decisions" about Mr. Bolton's nomination. Mr. Thune, a freshman, has clashed with the administration over a Defense Department plan to close a major military base in his state.
One Democrat, Senator Barbara Boxer of California, had sought to block a Senate vote on Mr. Bolton, saying she would oppose any vote until the State Department provided documents related to the nomination that the department has so far refused to hand over.
On Tuesday afternoon, however, a spokeswoman for Ms. Boxer said she had decided to lift a hold on Mr. Bolton's nomination. Ms. Boxer's spokeswoman said she would join with Senator Joseph R. Biden Jr. of Delaware in agreeing to a Republican plan to move toward a vote on Mr. Bolton after allowing up to 40 hours of debate.
It appeared unlikely that any Senate Democrat would try to use a filibuster to block a vote, Senate Democratic officials said.