President Bush is nominating Undersecretary of State John Bolton to be the next U.S. ambassador to the United Nations. Mr. Bolton, a favorite of political conservatives, has been the Bush administration's top arms control official.

Mr. Bolton has taken a hard line towards talks with Iran and North Korea on their nuclear programs, and at times has been an outspoken critic of the United Nations itself.

But at a joint press appearance with the nominee, Secretary of State Condoleezza Rice said the Bush administration is committed to the success of the United Nations, and views it as an important component of its diplomacy.

She called Mr. Bolton a tough-minded diplomat with a proven track record of multilateralism, and said some of the most effective American envoys to the United Nations have been those who have not been reluctant to speak their minds:

"Through history, some of our best ambassadors have been those with the strongest voices; Ambassadors like Jeanne Kirkpatrick and Daniel Patrick Moynihan," she said. "John Bolton is personally committed to the future success of the United Nations, and he will be a strong voice for reform at a time when the United Nations has begun to reform itself."

The secretary praised Mr. Bolton's record over the past four years as Undersecretary of State, citing his lead role in the Proliferation Security Initiative, an informal U.S.-led grouping of some 60 countries committed to joint efforts to stop the trafficking of weapons of mass destruction.

She also noted his record in past Republican administrations and said as an assistant secretary of state in the early 1990s, he was the principal architect of the U.S.-led effort to overturn the controversial 1975 U.N. General Assembly resolution equating Zionism with racism.

In his remarks at the event, Mr. Bolton referred to the repeal of the resolution on Zionism as one of the highlights of his professional career, and said it had removed the greatest stain on the United Nations' reputation.

While acknowledging his past criticism of the U.N., Mr. Bolton said his record over the years nonetheless demonstrates clear support for effective multilateral diplomacy through the international system.

"I have consistently stressed in my writings that American leadership is critical to the success of the U.N., an effective U.N., one that is true to the original intent of its charter's framers," he said. "This is a time of opportunity for the U.N., which likewise requires American leadership to achieve successful reform."

State Department officials, defending the choice of Mr. Bolton, said dealing forthrightly with U.S. concerns about the United Nations is essential to continuing American public support for the organization.

They said Secretary Rice, in the hours before the nomination was announced, telephoned U.N. Secretary-General Kofi Annan, the foreign ministers of key U.S. allies and leaders of both parties in Congress to inform them of the decision.

The Bolton nomination will require Senate confirmation, which is expected though there is likely to be Democratic criticism of Mr. Bolton in confirmation hearings.

In a possible preview of Senate debate, Democratic Senator John Corzine of New Jersey said he was deeply disappointed over the nomination of Mr. Bolton.

Mr. Corzine described Mr. Bolton as a leading foreign policy hardliner, and said he was responsible as much as any member of the Bush administration for needless confrontations with other countries and the international isolation he said had plagued the president's first term.