The White House had said that Mr. Bush would wield his veto pen shortly after 6 p.m. Eastern time, following his return from Tampa, Fla., where he took part in a conference of the Central Command, which oversees United States military operations in Iraq and Afghanistan.

“Everyone in this room knows the consequences of failure in Iraq,” Mr. Bush said at the conference, “and then we should also appreciate the consequences of success, because we’ve seen them before.” Look to Germany and Japan, once America’s mortal enemies but now its allies, he said.

Image Senate Majority Leader Harry Reid and Speaker of the House Nancy Pelosi signed the Iraq Supplemental Conference Report and sent it to President Bush Tuesday. Credit... Yuri Gripas/Reuters

“These are difficult times,” Mr. Bush said. “These are tough times. These are the times that test the resolve of free people.”

Democrats have been saying for months that the “free people” voted last November for a change of course in Iraq, and they pressed the attack today. “Today is the fourth anniversary of what I consider to be one of the most shameful episodes in American history,” said Senator Hillary Rodham Clinton of New York, a Democratic candidate for president. “Never before in our history has a president said ‘mission accomplished’ when the mission had barely begun.”

Representative Rahm Emanuel of Illinois, chairman of the House Democratic Caucus, said: “In the four years since the president claimed ‘mission accomplished’ in Iraq, America has lost thousands of young lives and spent hundreds of billions of dollars.”

The White House quickly pointed out that Mr. Bush never spoke the words “mission accomplished” in his speech. Rather, they were displayed on a banner aboard the aircraft carrier U.S.S. Abraham Lincoln, where Mr. Bush landed shortly after the Iraqi dictator Saddam Hussein was toppled by the American-led invasion.