Democratic presidential hopeful Sen. Barack Obama, D-Ill., waits backstage before a rally Friday in Sioux Falls, S.D.

Watertown, S.D. ? 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 meetings 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.”