"Our enemies will test the new president early," Lieberman, I-Conn., told Face The Nation host Bob Schieffer. "Remember that the truck bombing of the World Trade Center happened in the first year of the Clinton administration. 9/11 happened in the first year of the Bush administration."
Lieberman nonetheless distanced himself from remarks by McCain chief strategist Charlie Black, who came under criticism for suggesting in an interview that McCain's election chances would be improved if a terrorist attack occurred before November.
"Sometimes even the best of them say things that are not what they intended to say," Lieberman said. "Certainly the implications there I know were not what Charlie intended. And he apologized for it. Senator McCain said he didn't agree. And, of course, I feel the same way.
"But here's the point. We're in a war against Islamist extremists who attacked us on 9/11. They've been trying to attack us in many, many ways since then."
A former Democratic nominee for vice president, Lieberman endorsed McCain for president because, he says, the Democratic Party he joined in the early 1960s is not reflected by the party's current leadership.
He also said that he feels McCain is better prepared to be commander in chief than Barack Obama. "[McCain] knows the world," Lieberman said. "He's been tested. He's ready to protect the security of the American people."
Lieberman also assailed Obama and fellow Senators who called for a timetable of withdrawal of U.S. troops from Iraq and opposed the "surge" of additional U.S. forces pushed forth by President Bush.
"It's now working," Lieberman told Schieffer. "If we had done what Senator Obama asked us to do for the last couple of years, today Iran and al Qaeda would be in control of Iraq. It would be a terrible defeat for us and our allies in the Middle East and throughout the world. Instead, we've got a country that's defending itself, that's growing economically, where there's been genuine political reconciliation, and where Iran and al Qaeda are on the run. And that's the way it ought to be."
However, McCain's readiness was disputed by retired General Wesley Clark, who is backing Obama for president, despite McCain's storied military experience in Vietnam. "Well, I don't think riding in a fighter plane and getting shot down is a qualification to be president," he said.
"And I think what we need to do, Bob, is we need to stop talking about the old politics of left and right, and we need to pull together and move the country forward. And I think that's what Barack Obama will do.
"Because in the matters of national security policymaking, it's a matter of understanding risk. It's a matter of gauging your opponents and it's a matter of being held accountable. John McCain's never done any of that in his official positions. I certainly honor his service … But he hasn't held executive responsibility."
Read the full "Face the Nation" transcript here.