President Bush sidestepped the U.S. Senate on Monday and installed controversial nominee John Bolton as ambassador to the United Nations, saying the post was "too important to leave vacant any longer."

Speaking at the White House, Bush said he was sending Bolton, a 56-year-old lawyer, to the United Nations with his "complete confidence."

The appointment constituted what is known as a recess appointment. It ended a five-month impasse with Senate Democrats who had accused the conservative Bolton of twisting intelligence to suit a hawkish ideology and of abusing subordinates.

Bush has the power to fill vacancies without Senate approval while Congress is in recess. Under the Constitution, the recess appointment will last until after the Senate adjourns in the fall of next year.

Speaking at a White House Roosevelt Room ceremony flanked by the mustachioed Bolton and Secretary of State Condoleezza Rice, Bush said that "a majority of U.S. senators agree that he is the right man for the job. Yet, because of partisan delaying tactics by a handful of senators, John was unfairly denied the up or down vote that he deserves."

In a brief acceptance speech, Bolton, who has a long history of criticizing the United Nations, said he was "profoundly honored, indeed humbled by the confidence" the president had shown in him.

Bush had refused to give up on Bolton even though the Senate had twice voted to sustain a filibuster against him.

Senate Democrats quickly criticized the president's move.

Sen. Edward M. Kennedy (D-Mass.), called it a "devious maneuver" that only "further darkens the cloud over Mr. Bolton's credibility."

"The abuse of power and the cloak of secrecy from the White House continues," Kennedy said. "It's bad enough that the administration stonewalled the Senate by refusing to disclose documents highly relevant to the Bolton nomination. It's even worse for the administration to abuse the recess appointment power by making the appointment while Congress is in this five-week recess."

Sen. Christopher Dodd of Connecticut, a senior Democrat on the Senate Foreign Relations Committee, said: "The president has done a real disservice to our nation by appointing an individual who lacks the credibility to further U.S. interests at the United Nations. I will be monitoring his performance closely to ensure that he does not abuse his authority as he has in the past."

In New York, U.N. Secretary General Kofi Annan said he looked forward to working with Bolton as he does with the 190 other ambassadors at the United Nations. About the recess appointment, Annan said: "I think it is the president's prerogative, and the president has decided to appoint him through this process."

Bolton, an outspoken conservative, triggered controversy from the moment Bush nominated him March 8.

State Department officials accused him of berating career officials and analysts who challenged his views, and of selectively choosing intelligence to support his assertions about the dangers posed by Cuba and other nations.

When a Republican on the Foreign Relations Committee, Sen. George V. Voinovich (Ohio), decided to oppose Bolton, the nomination moved to the full Senate with no recommendation.

Since then, the impasse focused on Democrats' demands to see two sets of documents related to Bolton's State Department work. One involved national security intercepts of conversations.

Democrats wanted to know whether Bolton was seeking secret information on rivals in the intelligence and foreign policy communities. The other documents involved Syria and questions of whether Bolton misled lawmakers about his role in compiling them.

Bolton -- who lost the title of undersecretary of state June 1 when his successor, Robert Joseph, was sworn in -- has spent the past four months in a transition suite at the State Department, and colleagues said he continued to ready himself for the ambassadorship.

Two months ago, while his confirmation was in trouble, Bolton began efforts to double the office space reserved within the State Department for the ambassador to the United Nations, according to three senior department officials who were involved in handling the request.

Previous ambassadors have kept a small staff in Washington in a modest suite. Bolton told several colleagues he needed more space and a larger staff in Washington because, if confirmed, he intended to spend more time here than his predecessors did. "Bolton isn't going to sit in New York while policy gets made in Washington," the administration source said, speaking on the condition of anonymity because the source lacked authorization to discuss this on the record.

Washington Post staff writers Robin Wright, Charles Babington and Dafna Linzer contributed to this report