WASHINGTON — President Bush's choice for U.N. ambassador received a serious setback yesterday when the Senate Foreign Relations Committee agreed to delay a vote on his nomination to investigate fresh allegations of improper conduct.

The decision came after Sen. George Voinovich, R-Ohio, shocked fellow Republicans by saying he wanted more time to study the charges against John Bolton, the State Department's top arms-control official.

"I've heard enough today to give me some real concern," Voinovich said. "I don't feel comfortable voting today."

The nomination would have been blocked from reaching the full Senate by a 9-9 vote if Voinovich had joined the Democrats in opposing Bolton. All 10 Republicans on the committee had been expected to vote for Bolton, an outspoken critic of the United Nations and a past proponent of go-it-alone U.S. foreign policy.

Voinovich's unexpected objection took the wind out of a forceful effort by the chairman, Sen. Richard Lugar, R-Ind., to overrule Democrats' demands for more time and hold a vote. It also gave Sen. Chuck Hagel, R-Neb., an opening in which to state his reservations about a quick vote.

Lugar then agreed to put off the vote until after the Senate returns from a recess next month in return for the Democrats' promise to abide by the date he sets. Until then, Republican and Democratic committee staff members are working together to examine new allegations.

In arguing for a delay, the Democrats said they had received new allegations of improper behavior by Bolton since a hearing last week in which a former senior State Department official said Bolton asked him to fire an intelligence analyst who disputed his views on Cuba's biological-warfare capabilities.

The Democrats also established that Bolton sought to have an analyst on the National Intelligence Council, an advisory group to the CIA director, replaced for the same reasons. Bolton didn't dispute the allegation.

Sen. Joe Biden, D-Del., said his staff had learned of at least two other episodes in which Bolton had sought to penalize junior bureaucrats.

A former contractor for the U.S. Agency for International Development also has come forward with charges that Bolton had harassed her and spread falsehoods about her while he worked as a lawyer for a private firm, Biden said.

Biden also questioned Bolton's credibility, saying his versions of the episodes involving the intelligence analysts differed considerably from versions given by the analysts and their superiors.

Bolton, 56, has served four years as arms-control chief at the State Department, but he is not a diplomat by training. He was an assistant attorney general under the first President Bush and held other government jobs during the Reagan administration.

Bolton's background was provided by The Associated Press.