A Republican U.S. senator has urged his colleagues to reject President Bush's nominee for ambassador to the United Nations, John Bolton.

The appeal from Senator George Voinovich of Ohio comes as the Senate prepares to begin floor debate on Mr. Bolton's nomination, possibly Wednesday.

The majority Republicans predict the Senate will confirm Mr. Bolton, who is currently undersecretary of state for arms control.

But in a letter sent to all 99 Senate colleagues, Senator Voinovich said naming Mr. Bolton ambassador could jepoardize the nation's ability to win the war on terror and advance U.N. reform.

Democrats who also oppose the nominee have criticized him as being too blunt and inflexible for the U.N. job.

Mr. Bolton retains strong backing from the White House, which says he has the ability and experience to get things done at the United Nations.