WASHINGTON — President Bush will not relent in his defense of his nominee for U.N. ambassador, John Bolton, despite unwavering opposition from Democrats who view Mr. Bolton as too combative for international diplomacy, aides said yesterday.

Two of Mr. Bush’s top advisers said the White House is not backing down from a fight to win Senate approval for Mr. Bolton to continue in the job. Mr. Bush gave Mr. Bolton the job temporarily in August 2005, while Congress was in recess. That appointment will expire when Congress adjourns, no later than January.

White House counselor Dan Bartlett said Mr. Bolton has done a remarkable job. “He’s proven the critics wrong on all the charges they’ve leveled against him,” Mr. Bartlett said. “So let’s have a conversation about it. We’ll see.”

The White House resubmitted Mr. Bolton’s nomination on Thursday, though it has languished in the Senate for more than a year. Finding a replacement for Mr. Bolton would come at a sensitive time for the Bush administration. It is counting heavily on U.N. diplomacy to help confront North Korea and Iran over their nuclear programs and to end fighting in Sudan’s Darfur region.

With Democrats capturing control of the next Congress, Mr. Bolton’s chances of winning confirmation appear slim at best. Last week, the incoming chairman of the Senate Foreign Relations Committee, Senator Biden, a Democrat of Delaware, said he saw “no point in considering Mr. Bolton’s nomination again.”