The Senate is expected to vote on John R. Bolton's nomination before leaving for a Memorial Day break, and Republicans said they are confident he will be confirmed.
The vote would end weeks of wrangling over whether Bolton mistreated co-workers or took liberties with government intelligence.
"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," Voinovich wrote to all 99 other senators. "I worry that Mr. Bolton could make it more difficult for us to achieve the important U.N. reforms needed to restore the strength of the institution."
Voinovich first forced a delay in Bolton's confirmation last month and then brokered an unusual compromise that sent Bolton's nomination to the full Senate without the customary recommendation from a Senate committee.
Voinovich echoed Democratic objections to Bolton, saying his conduct and temperament make him ill-suited for a sensitive diplomatic post. Voinovich planned to make a lengthy case against Bolton on the Senate floor.
Republicans have expressed confidence that Bolton will win easy confirmation on the floor, where the party holds a 55-45 effective majority, but there were signs Tuesday that other Republicans may defect with Voinovich.
Sen. John Thune, R-S.D., is undecided, spokesman Alex Conant said. It was the first time Thune's name was mentioned along with about five other Republicans as a possible no vote.
Democrats said they have not ruled out a procedural tactic to postpone a vote, but several said it is unlikely.
"There is no desire for a filibuster," Sen. Chris Dodd, D-Conn., said Tuesday.