WASHINGTON (CNN) -- Five years after he green-lighted the U.S.-led invasion of Iraq, President Bush faced strikingly low approval ratings as he reaffirmed his commitment to "accept no outcome but victory" in the war.

Just 31 percent of Americans approve of how President Bush is handling his job, according to a poll released Wednesday, the anniversary of the start of the conflict in 2003.

Sixty-seven percent of those questioned in a CNN/Opinion Research Corporation survey disapprove of the president's performance.

The 31 percent approval number is a new low for Bush in CNN polling and is 40 points lower than the president's number at the start of the Iraq war.

"Bush's approval rating five years ago, at the start of the Iraq war, was 71 percent, and that 40-point drop is almost identical to the drop President Lyndon Johnson faced during the Vietnam War," CNN polling director Keating Holland said.

"Johnson's approval rating was 74 percent just before Congress passed the Gulf of Tonkin resolution in 1964, which effectively authorized the Vietnam War. Four years later, his approval was down to 35 percent, a 39-point drop that is statistically identical to what Bush has faced so far over the length of the Iraq war," he said.

But there was no sign that the conflict would end soon.

During a speech at the Pentagon Wednesday, the president called the debate over Iraq "understandable" but insisted that a continued U.S. presence in the region was crucial.

"Defeating this enemy in Iraq will make it less likely we will face this enemy here at home," he said.

"We're helping the people of Iraq establish a democracy in the heart of the Middle East. A free Iraq will fight terrorists instead of harboring them." Watch Bush speak on the fifth anniversary of the war »

Not far away from the Pentagon, where the president was speaking, voices called for an end to the conflict.

Several hundred anti-war protesters marched through Washington, splattering red paint on government and defense contractors' offices and occasionally scuffling with police.

Protesters, including many veterans, demanded the arrests of President Bush, Vice President Dick Cheney and Secretary of State Condoleezza Rice as war criminals. Watch the protests in Washington »

Others hurled balloons full of paint at a military recruiting station and smeared it on and outside buildings housing defense contractors Bechtel and Lockheed Martin.

At least 31 people were arrested after crossing police lines outside the Internal Revenue Service building on Pennsylvania Avenue, protest organizer Freida Berrigan said.

Other protests took place in San Francisco, where 115 people were arrested and released after being cited for misdemeanors such as trespassing, resisting arrest and blocking an intersection, said Sgt. Steve Mannina, a police spokesman.

The legacy of the war

Bush ordered U.S. troops into Iraq on March 19, 2003, after months of warnings that Iraqi leader Saddam Hussein was hiding stockpiles of chemical and biological weapons and efforts to build a nuclear bomb.

U.N. weapons inspectors found no sign of banned weapons before the invasion, and the CIA later concluded that Iraq had dismantled its weapons programs in the 1990s.

Almost 4,000 Americans have died in Iraq since then, and estimates of the Iraqi toll range from about 80,000 to 150,000 or more.

Almost 160,000 U.S. troops remain in Iraq, and the war has cost U.S. taxpayers about $600 billion, according to the House Budget Committee.

The president's approval rating has been below 35 percent since October and has not cracked 40 percent since September 2006. Watch CNN's Bill Schneider discuss Bush's poll numbers »

Still, Bush's approval number is still better than the lowest number for his father, George H.W. Bush, who bottomed out at 29 percent in July 1992; Jimmy Carter, who fell to 28 percent in June 1979; Richard Nixon, at 24 percent in July and August 1974; and Harry Truman, who dipped to 22 percent in 1952.

"Lame-duck presidents presiding over unpopular wars or struggling economies have gotten low approval ratings in the past," Holland said.

"By contrast, lame ducks like Ronald Reagan, Dwight Eisenhower and Bill Clinton had robust approval ratings in their final years in office, but each one was presiding over good economic times and a country at peace."

The CNN/Opinion Research Corporation poll was conducted by telephone with 1,019 adult Americans from Friday through Sunday.

The survey's sampling error is plus or minus 3 percentage points. E-mail to a friend

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