WASHINGTON – Lacking the votes to keep his job, embattled U.N. Ambassador John Bolton said Monday he would resign, a defeat for a chagrined President Bush who had clung to hopes of Senate confirmation.

Bolton got the position in August 2005, appointed by Bush when Congress was in recess. With that temporary assignment about to expire, and his long fight for confirmation going nowhere, Bolton made it official.

He handed in a resignation letter that did not mention the political fight behind it. It said simply: “I have concluded that my service in your administration should end when the current recess appointment expires.”

“I accepted. I’m not happy about it,” Bush said Monday afternoon in the Oval Office, with Bolton at his side. Bush did not name a replacement, and officials offered no timetable for an announcement.

The setback for the White House seemed to put a hold on the postelection talk of bipartisanship.

Bush considered Bolton a strong voice as the U.N. dealt with crises in Iraq, Lebanon, North Korea and other complex matters around the world. Bolton also pushed the administration’s effort to reform the United Nations.

But Democrats opposed Bolton, whom they viewed as a brusque, ill-suited diplomat. Some Republicans helped scuttle his nomination, including moderate Republican Sen. Lincoln Chafee of Rhode Island.

The president had stinging words for them.

“They chose to obstruct his confirmation, even though he enjoys majority support in the Senate, and even though their tactics will disrupt our diplomatic work at a sensitive and important time,” Bush said in a statement. “This stubborn obstructionism ill serves our country.”

Democrats, though, said Bolton’s resignation signaled a fresh start.

“Hopefully this change marks a shift from the failed go-it-alone strategies that have left America less safe,” said the incoming Senate majority leader, Democrat Harry Reid of Nevada.

“With the Middle East on the verge of chaos and the nuclear threats from Iran and North Korea increasing, we need a United Nations ambassador who has the full support of Congress and can help rally the international community to tackle the serious threats we face,” said Sen. John Kerry, D-Mass.

Bolton’s nomination had languished in the Senate Foreign Relations Committee for more than a year. The White House made a renewed push for him after Democrats won control of Congress in November.

But it became clear that “there were not going to be the votes” to get the nomination out of committee, said White House spokesman Tony Snow. He suggested the entire confirmation process was broken.