A conservative group backed by two fundraisers for President Bush has launched an ad attacking Sen. John F. Kerry (D-Mass.) that shows pictures of Osama bin Laden, hijacker Mohamed Atta, Russian hostage-takers, the Madrid bombers and the World Trade Center.

"It's the Willie Horton ad of 2004," Kerry spokesman Phil Singer said last night, referring to the racially charged ad about a convicted murderer used against Gov. Michael S. Dukakis in the 1988 campaign. The Democratic nominee rushed out a response ad accusing the Bush campaign of "un-American" tactics, although the ad's sponsor, the Progress for America Voter Fund, is legally independent of the Bush campaign.

The unusual weekend battle marks the harshest exchange yet in a political season marked by relentlessly negative commercials.

Bush campaign spokesman Steve Schmidt repeated the president's call for all "527" advocacy groups to take down their ads but made no effort to distance Bush from the message. "Senator Kerry has played politics with the war on terror at every turn," Schmidt said, "and his chronic vacillation . . . will make the world more dangerous and America less safe."

The attack ad, picturing some of the world's most notorious terrorists, says: "These people want to kill us. They killed hundreds of innocent children in Russia, 200 innocent commuters in Spain and 3,000 innocent Americans.

"John Kerry has a 30-year record of supporting cuts in defense and intelligence and endlessly changing positions in Iraq. Would you trust Kerry against the fanatic killers? President Bush didn't start this war, but he will finish it."

While Kerry has voted for some military cutbacks as a senator, his campaign says he has supported $4.4 trillion in defense spending since 1985, including a sizable increase in 2002.

Progress for America, which is represented by the former Bush campaign lawyer, Ben Ginsberg, has raised at least $14 million. More than two-thirds of it has come from two Californians, Alex Spanos and Dawn Arnall, who are major Bush and GOP fundraisers. Singer said the group is clearly "linked" to Bush. Last week, the fund ran a mocking ad of Kerry windsurfing that was similar to a Bush campaign spot.

The Kerry counterattack ad is built around quotes from a New York Times editorial yesterday criticizing Bush campaign tactics.

The ad begins: " 'Despicable politics.' 'An Un-American way to campaign.' The latest Bush-Cheney attacks against John Kerry. George Bush and Dick Cheney are using the 'appalling' and 'divisive' strategy of playing politics with the war on terror. A strategy that 'undermines the efforts . . . to combat terrorists in America' and puts George Bush's 'own ambition ahead of the national good.' " The spot calls on Bush to "stop dividing America."

While the Progress for America ad is running only in Wisconsin and Iowa, the Kerry camp seems determined not to repeat its slow response to the Swift Boat Veterans for Truth, whose initial ads ran in three states. Former Democratic senators Max Cleland (Ga.) and Bob Kerrey (Neb.) issued statements condemning the new ad.