Democratic presidential candidate John F. Kerry charged today that President Bush has made it easier for terrorists to harm Americans by failing to push for renewal of a ban on sales of assault weapons.

In an appearance in Washington where he picked up the endorsement of a coalition of police unions, the Massachusetts senator accused Bush of breaking his word to the nation's police officers and siding instead with the powerful gun lobby.

The ban, enacted in 1994 under the Clinton administration, expires today. The National Rifle Association, an influential lobbying group representing gun owners and manufacturers, has strongly opposed reauthorizing the 10-year ban, arguing that it has been ineffective and mainly "cosmetic."

Addressing an audience at the Thurgood Marshall Center in Washington, Kerry said police officers "are tired of a president who takes cops off the streets with one hand and puts . . . military assault weapons back on with the other." He said Bush promised when he ran for president in 2000 that he would extend the assault weapons ban.

"But when it came time to do it . . . George Bush's powerful friends in the gun lobby asked him to look the other way, and he couldn't resist, and he said, sure," Kerry said. "And so tomorrow, for the first time in 10 years, when a killer walks into a gun shop, when a terrorist goes to a gun show somewhere in America, when they want to purchase an AK-47 or some other military assault weapon, they're going to hear one word: sure. Today, George Bush chose to make the job of terrorists easier and make the job of America's police officers harder, and that's just plain wrong."

Kerry was endorsed today by the National Association of Police Organizations, which includes more than 2,000 police unions and associations.

In remarks at the same event, District Del. Eleanor Holmes Norton (D) said she had been informed that the House "soon will schedule a vote on a bill to totally repeal all of D.C.'s gun laws." She said that in a year that would be remembered for the large number of children killed by guns in Washington, "we in the District can only be outraged that the House would twice expose our children to death by gunshot, first by failing to reauthorize the assault weapons ban, and then by repealing all of the city's gun laws."

Asked about the assault weapons ban today, White House spokesman Scott McClellan dismissed what he called "another false attack from Senator Kerry" and said that "the best way to deter and combat violence committed with guns is to vigorously enforce our laws." He said prosecutions of gun-related crimes are up 68 percent under the Bush administration and violent crime "is at its lowest rate in 30 years."

As for whether the assault weapons ban has been effective, McClellan said, "I think that's a question that other people are looking at, still."

He denied that the president was being disingenuous by publicly backing reauthorization of the ban but not lobbying legislators to pass such a bill, as he has on other legislation that he supports. McClellan said it was "ridiculous" to assert that the president was happy to let the ban lapse.

Pressed repeatedly to name one legislator Bush has called or one speech he has given to urge renewal of the assault weapons ban, McClellan declined to answer directly.

"His position has been very well-known," McClellan said. "But he does not set the legislative timetable." Republican leaders in Congress have made it clear that the reauthorization would not come up for a vote, McClellan noted.

The issue set off a statistical duel between the Bush and Kerry campaigns.

In a statement, the campaign of Bush and Vice President Cheney said that the "violent crime victimization rate" has dropped 21 percent during Bush's first three years in office compared to the last two years of the Clinton administration, reaching a 30-year low. The rate, a measure of victimizations per 1,000 U.S. residents age 12 and older, includes rape, sexual assault, robbery, aggravated assault and simple assault, but not homicide.

In his speech today, however, Kerry said that murders, gang-related deaths and violent deaths at schools have risen under the Bush administration, which he accused of paying for massive tax cuts in part by gutting programs such as the COPS initiative. The program, which Kerry promoted as senator to prevent crime by putting 100,000 more police officers on the nation's streets, was part of the 1994 crime bill. The bill included $8.8 billion for hiring more police and banned the sale of 19 types of assault weapons.

Kerry's campaign charges that Bush has sought to eliminate more than 80 percent of COPS funding in each of his budgets and that this year he proposed cutting the program to $97 million from $756 million.

Kerry said today that the crime bill's measures have been successful, resulting in "record drops in violent crime all over America" and a decrease of 65 percent in assault weapon-related crimes. But under the Bush administration, Kerry said, "the most violent crimes have made a comeback," with 800 more murders a year, gang-related deaths up 40 percent and violent deaths at high schools at a five-year high.

According to a report issued Sunday by the Justice Department, the violent crime victimization rate dropped 14 percent from 2000-2001 to 2002-2003, continuing a decade-long trend in which it fell by 55 percent.

The department's statistics show that the rate plunged steadily from 51.2 violent crimes per 1,000 people in 1994 to 22.8 in 2002, then practically leveled off, reaching 22.3 in 2003. Murder is not included in the rate because it is based on surveys of crime victims. But a separate report by the FBI found that murders rose by 1.3 percent between 2002 and 2003.

With the assault weapons ban set to expire, other gun control advocates urged Congress to take up the issue.

Sarah Brady, the wife of former White House press secretary James Brady, who was seriously wounded in a 1981 assassination attempt on President Ronald Reagan, said on CBS's "Early Show" today that "there is nothing cosmetic at all about this law."

Asked why the ban was being allowed to lapse tonight, she said, "It's purely political, and the real onus fell on President George W. Bush."

She said Bush "tried to say on one hand he's for the ban, but he has done absolutely nothing, exerted no leadership to be sure that the House and the Senate would bring it up and pass it." Brady said the president and congressional leaders "simply do not want to face this and are looking to the endorsement of special interest groups more than they are to public safety and law enforcement."

Rep. C.L. "Butch" Otter (R-Idaho), a staunch opponent of the ban, said in a statement that it was "nothing more than a sop to antigun liberals." The statement said the ban "provided only the illusion of reducing gun violence" while doing "real damage to our liberties."