In naming Bolton to the high-profile post, Bush did an end run on congressional critics who said Bolton was temperamentally ill-suited for the job, primarily because of allegations that he had a volatile temper and that he tried to manipulate intelligence that contradicted the administration's policies.

Bush used a constitutional provision that allows presidents to make temporary appointments without Senate approval during a congressional recess, after Senate Democrats had refused to vote on Bolton's candidacy unless they received additional internal documents related to his tenure at the State Department.

Utah's senators supported Bush's decision, saying Bolton is needed at the United Nations.

"John Bolton's nomination was turned into a circus by a handful of senators who - once again - abused the filibuster to prevent a qualified man to serve the president," said Sen. Orrin Hatch. "This is a man that is so experienced that he's been confirmed by the Senate numerous times - including by those who now, conveniently, find him unacceptable."

Sen. Bob Bennett said he believes Bolton will make significant contributions to the United Nations.

"As our country and the world continue to face critical times, it is imperative for the United States to have permanent representation in the United Nations," Bennett said.

Critics say Bolton's aggressive style will make it harder for the United States to nurture alliances on important issues that require broad international coalitions such as combating Islamic terrorism and halting nuclear fuel production in North Korea and Iran. Relations with allies already have frayed over the war in Iraq and a Bush administration approach to foreign policy that many nations consider disdainful of their interests.

Democrats responded to Bolton's appointment with vociferous criticism. Senate Minority Leader Harry Reid, D-Nev., denounced the appointment as ''the latest abuse of power by the Bush administration,'' adding that Bolton would arrive at the United Nations ''with a cloud hanging over his head'' because he could not win confirmation.

Under the rules for recess appointments, Bolton's tenure will lapse with the seating of the next Congress in January 2007 if he is not confirmed before then.

At a White House ceremony announcing the appointment, Bush blamed ''partisan'' actions by Democrats for the impasse over the nomination, which he made five months ago.

''This post is too important to leave vacant any longer, especially during a war and a vital debate about U.N. reform,'' Bush said.

He added that Bolton would go to the United Nations with his full confidence.

The annual session of the U.N. General Assembly begins in mid-September, shortly after the Senate returns to work from its August recess. After a morning announcement ceremony at the White House, Bolton flew to the United Nations in New York to begin work.

United Nations Secretary-General Kofi Annan, responding to reporters' queries on whether Bolton's influence at the United Nations would be weakened because he was not confirmed, said, ''From where I stand, we will work with him, as the representative of the president and the government.''

Still, Annan added words of caution on the challenges Bolton would face in winning over other nations.

''An ambassador always has to remember that there are 190 others,'' Annan said.