Vice President Dick Cheney (search) on Tuesday evoked the possibility of terrorists bombing U.S. cities with nuclear weapons and questioned whether Sen. John Kerry (search) could combat such a threat, which the vice president called a concept "you've got to get your mind around."

"The biggest threat we face now as a nation is the possibility of terrorists ending up in the middle of one of our cities with deadlier weapons than have ever before been used against us — biological agents or a nuclear weapon or a chemical weapon of some kind to be able to threaten the lives of hundreds of thousands of Americans," Cheney said.

"That's the ultimate threat. For us to have a strategy that's capable of defeating that threat, you've got to get your mind around that concept," Cheney said.

Cheney, speaking to an invitation-only crowd as he began a bus tour through Republican strongholds in Ohio, said Kerry is trying to convince voters he would be the same type of "tough, aggressive" leader as Bush in the fight against terrorism.

"I don't believe it," the vice president said. "I don't think there's any evidence to support the proposition that he would, in fact, do it."

Cheney praised the recent elections in Afghanistan but said they don't mean the U.S. mission there is finished.

"Does that mean it's over now and we can walk away? No, it doesn't," he said. "This is three yards and a cloud of dust. There's no touchdown passes in this business. We'll stay as long as we need to help them train their own security forces, which we're doing actively so they can take over responsibility for their own security."

In a campaign appearance Monday in Johnstown, Pa., Cheney criticized rival vice presidential candidate John Edwards for going "overboard" in his comments about Kerry's support of unrestricted federal funding for stem cell research, which Bush and Cheney oppose. He also accused Edwards of giving people "false hope."

Edwards told supporters in Newton, Iowa, on Oct. 11, "If we do the work that we can do in this country, the work that we will do when John Kerry is president, people like Christopher Reeve will get up out of that wheelchair and walk again." The actor, a quadriplegic who became an advocate for stem cell research, had died a day earlier.

"I thought, frankly, the other day what John Edwards suggested when he made his comments about Christopher Reeve, that somehow if John Kerry were president, Christopher Reeve could get up out of his wheelchair and there all of his problems would be solved, I really thought was an inappropriate remark, especially given ... well, given the false hope it engendered," Cheney said.