WATERTOWN, S.D. (AP) - Barack Obama laid into John McCain on Friday for advancing a tough-guy foreign policy that he called "naive and irresponsible," serving notice that he's ready to launch a full-throttle challenge to the Republican presidential contender on international relations in the general election campaign.

Lumping McCain together with President Bush, Obama declared: "If they want a debate about protecting the United States of America, that's a debate I'm ready to win because George Bush and John McCain have a lot to answer for." He blamed Bush for policies that enhance the strength of terrorist groups such as Hamas and "the fact that al-Qaida's leadership is stronger than ever because we took our eye off the ball in Afghanistan," among other failings.

McCain agreed, at least, that there were huge differences between himself and Obama on foreign policy, and said he'd be happy to let the American people decide who was right.

"It would be a wonderful thing if we lived in a world where we don't have enemies. But that's not the world we live in. And until Senator Obama understands that reality, the American people have every reason to doubt whether he has the strength, judgment and determination to keep us safe," McCain said in a speech to the National Rifle Association in Louisville, Ky.

McCain rejected the naive comment, saying Obama should have known better, and added: "Talking, not even with soaring rhetoric, in unconditional meetings with the man who calls Israel 'a stinking corpse,' and arms terrorists who kill Americans, will not convince Iran to give up its nuclear program. It is reckless. It is reckless to suggest that unconditional neetings will advance our interests."

His campaign issued a statement accusing Obama of making a "hysterical diatribe."

The three-way dustup over foreign policy - Bush vs. Obama vs. McCain - began a day earlier, when Bush gave a speech to the Israeli Knesset in which he criticized those who believe the United States should negotiate with terrorists and radicals. Obama said Bush's criticism was directed at him, and took umbrage; the White House denied the president had Obama in mind; McCain said Obama must explain why he wants to talk with rogue leaders.

Obama continued the debate on Friday at a town-hall meeting in a livestock barn. He said he had planned to focus on rural issues during his swing through South Dakota, but felt compelled to answer the remarks from Bush and McCain.

"I'm a strong believer in civility and I'm a strong believer in a bipartisan foreign policy, but that cause is not served with dishonest, divisive attacks of the sort that we've seen out of George Bush and John McCain over the last couple days," he said.

Obama said McCain had a "naive and irresponsible belief that tough talk from Washington will somehow cause Iran to give up its nuclear program and support for terrorism."

Speaking of McCain and Bush together, he added: "They aren't telling you the truth. They are trying to fool you and scare you because they can't win a foreign policy debate on the merits. But it's not going to work. Not this time, not this year."

Obama vowed to turn the foreign policy debate back against Bush and McCain, rejecting the notion that Democrats critical of the war in Iraq are vulnerable to charges of being soft on terrorism. Meeting with reporters, he argued that tough-minded diplomacy and engagement with rivals have long coexisted, citing the foreign policies of former Presidents Kennedy, Nixon and Reagan.

"That has been the history of U.S. diplomacy until very recently," Obama said. "I find it puzzling that we view this as in any way controversial. This whole notion of not talking to people, it didn't hold in the '60s, it didn't hold in the '70s ... When Kennedy met with (Soviet leader Nikita) Khrushchev, we were on the brink of nuclear war."

He also noted that Nixon opened talks with China with the knowledge that Chinese leader Mao Zedong "had exterminated millions of people."

Laying down a marker for the fall campaign, Obama offered a challenge to the GOP nominee: "If John McCain wants to meet me anywhere, any time to have a debate about our respective policies ... that is a conversation I am happy to have."

Other Democrats accused McCain of hypocrisy Friday, saying the certain GOP presidential nominee had previously said he would be willing to negotiate with the militant Palestinian group Hamas.

McCain told reporters in West Virginia: "I made it very clear, at that time, before and after, that we will not negotiate with terrorist organizations, that Hamas would have to abandon their terrorism, their advocacy to the extermination of the state of Israel, and be willing to negotiate in a way that recognizes the right of the state of Israel and abandons their terrorist position and advocacy."

McCain said there was a "huge difference" between his own statements and Obama's willingness to negotiate with "sponsors of terrorist organizations."

"I'll let the American people decide whether that's a significant difference or not," he said. "I believe it is."

Obama said he has stated "over and over again that I will not negotiate with terrorists like Hamas."

Obama closed out his campaign day with a noisy rally in Sioux Falls before about 6,500 cheering backers, perhaps showing some of the strain of a long campaign day. "Thank you Sioux City," Obama said, as a roar greeted his entrance. He quickly corrected his reference to a nearby Iowa town.

"I've been in Iowa too long," said Obama, referring to his long campaign to win that state's leadoff caucuses in January.

Associated Press writer Glen Johnson in Louisville, Ky., contributed to this report.