WASHINGTON (AP) - President Bush acknowledged for the first time last night that he erred by not ordering a military buildup in Iraq last year and said he was increasing U.S. troops by 21,500 to quell the country's near-anarchy. "Where mistakes have been made, the responsibility rests with me," Bush said.

The buildup puts Bush on a collision course with the new Democratic Congress and pushes the American troop presence in Iraq toward its highest level. It also runs counter to widespread anti-war passions among Americans and the advice of some top generals.

In a prime-time address to the nation, Bush pushed back against the Democrats' calls to end the unpopular war. He said that "to step back now would force a collapse of the Iraqi government, tear that country apart and result in mass killings on an unimaginable scale."

"If we increase our support at this crucial moment and help the Iraqis break the current cycle of violence, we can hasten the day our troops begin coming home," Bush said. But he braced Americans to expect more U.S. casualties for now and did not specify how long the additional troops would stay.

In addition to extra U.S. forces, the plan envisions Iraq's committing 10,000 to 12,000 more troops to secure Baghdad's neighborhoods _ and taking the lead in military operations.

Even before Bush's address, the new Democratic leaders of Congress emphasized their opposition to a buildup. "This is the third time we are going down this path. Two times this has not worked," House Speaker Nancy Pelosi, (D-Calif.) said after meeting with the president. "Why are they doing this now? That question remains."

There was criticism from Republicans, as well. "This is a dangerously wrongheaded strategy that will drive America deeper into an unwinnable swamp at a great cost," said Sen. Chuck Hagel (R-Neb.) a Vietnam veteran and potential GOP presidential candidate

After nearly four years of bloody combat, the speech was perhaps Bush's last credible chance to try to present a winning strategy in Iraq and persuade Americans to change their minds about the unpopular war, which has cost the lives of more than 3,000 members of the U.S. military as well as more than $400 billion.

Senate and House Democrats are arranging votes urging the president not to send more troops. While lacking the force of law, the measures would compel Republicans to go on record as either bucking the president or supporting an escalation.

Usually loath to admit error, Bush said it also was a mistake to have allowed American forces to be restricted by the Iraqi government, which tried to prevent U.S. military operations against fighters controlled by the radical Shiite cleric Muqtada al-Sadr, a powerful political ally of Prime Minister Nouri al-Maliki. The president said al-Maliki had assured him that from now on, "political or sectarian interference will not be tolerated."

As Bush spoke for 20 minutes from the unusual setting of the White House library, the sounds of protesters amassed outside the compound's gates occasionally filtered through.

Bush's approach amounts to a huge gamble on al-Maliki's willingness - and ability - to deliver on promises he has consistently failed to keep: to disband Shiite militias, pursue national reconciliation and make good on commitments for Iraqi forces to handle security operations in Baghdad.

"Our past efforts to secure Baghdad failed for two principal reasons: There were not enough Iraqi and American troops to secure neighborhoods that had been cleared of terrorists and insurgents," the president said. "And there were too many restrictions on the troops we did have."

He said American commanders have reviewed the Iraqi plan "to ensure that it addressed these mistakes."

With Americans overwhelmingly unhappy with his Iraq strategy, Bush said it was a legitimate question to ask why this strategy to secure Baghdad will succeed where other operations failed. "This time we will have the force levels we need to hold the areas that have been cleared," the president said.

While Bush put the onus on the Iraqis to meet their responsibilities and commit more troops, he did not threaten specific consequences if they do not. Iraq has missed previous self-imposed timetables for taking over security responsibilities.

Bush, however, cited the government's latest optimistic estimate. "To establish its authority, the Iraqi government plans to take responsibility for security in all of Iraq's provinces by November," the president said.

Still, Bush said that "America's commitment is not open-ended. If the Iraqi government does not follow through on its promises, it will lose the support of the American people and it will lose the support of the Iraqi people. Now is the time to at."

Resisting calls for troop reductions, Bush said that "failure in Iraq would be a disaster for the United States. .. A democratic Iraq will not be perfect.