Oct. 9, 2008 -- Sen. John McCain acknowledged he is trailing Sen. Barack Obama in the 2008 presidential race, but says he long ago got used to being an "underdog" and willl keep fighting because he insists the Democratic nominee has neither the experience nor the candor to win the White House.

"This is a tough campaign," McCain told ABC News' Charlie Gibson in an exclusive interview. "I'm the underdog. I've always been the underdog from the beginning."

And in a surprising tactic, McCain repeatedly invoked Democratic Sen. Hillary Clinton -- an unpopular figure among conservative Republicans -- while defending his campaign's most controversial strategies: spending $300 billion to buy up peoples' mortgages and linking Obama to 1960s anti-war radical William Ayers.

McCain was most heated when pressing his campaign's attempts to connect Obama to Ayers, a co-founder of the Weather Underground, a Vietnam-era group that executed domestic bombings and plotted attacks on the U.S. Capitol and the Pentagon.

"[Ayers] wasn't a guy in the neighborhood. [Obama] launched his political career in his living room, in Mr. Ayers' living room. And I don't care about two washed-up old terrorists that are unrepentant about trying to destroy America. But I do care, and Americans should care, about his relationship with him and whether he's being truthful and candid about it," McCain said.

Obama says the personal attacks levied against him by the McCain campaign, particularly references to Ayers, are an attempt to "score cheap political points."

"Why don't we just clear it up right now," Obama told "World News" anchor Gibson in an exclusive interview on Wednesday. "I'll repeat again what I've said many times. This is a guy who engaged in some despicable acts 40 years ago when I was eight years old. By the time I met him, 10 or 15 years ago, he was a college professor of education at the University of Illinois . . . And the notion that somehow he has been involved in my campaign, that he is an adviser of mine, that . . . I've 'palled around with a terrorist', all these statements are made simply to try to score cheap political points."

But McCain was unrepentant when asked on Thursday by Gibson if Ayers is a "critical issue or factor in this campaign."

"I think it's a factor about Senator Obama's candor and truthfulness with the American people," McCain replied, before adding, "I don't care about Mr. Ayers, who on Sept. 11, 2001, said he wished he'd have bombed more. I don't care about that. I care about [Obama] being truthful about his relationship with him. And Americans will care."

Obama said on Wednesday to ABC News that the McCain campaign is making personal attacks "the centerpiece of the discussion in the closing weeks of a campaign where we are facing the worst economic crisis since the Great Depression and we're in the middle of two wars."

"I think that makes very little sense, not just to me but to the American people," Obama said.

McCain demurred when asked whether Obama's character or lack of candor disqualifies him to be president.

"I'll leave that up to the American people. But I have every right to insist that he be candid and truthful with the American people. And he needs to be asked about it, and he needs to be forthcoming," McCain said.

Obama had noted that McCain didn't raise the issue "to my face" in their debate on Tuesday, but McCain told Gibson he did not raise the Ayers argument during the debate because "it didn't come up in the flow of conversation."

But McCain told Gibson he felt comfortable with the subject as a focus in the last days of the campaign.

"I think it's something that needs to be examined. Sen. Clinton said it should be examined during their primary and it never was," McCain said.

McCain Questions Obama's Experience, Candor as Election Day Nears

McCain, who has thus far resisted engaging in some of the political attacks more often seen from his surrogates or Republican vice presidential nominee Sarah Palin on the campaign trail, repeatedly hinged his first direct discussion of Ayers on Obama's primary rival, Sen. Hillary Clinton, D-N.Y.

"Sen. Clinton in their debates said that the American people didn't know enough about him, including his relationship with Mr. Ayers. That's what she said. And I agree with that," McCain said, rejecting the suggestion that Obama has been "thoroughly vetted" over the course of a near two-year battle for the White House.

"Does he have the experience and the knowledge and judgment and has he made the right decisions?" McCain asked in the Gibson interview, "and has he told -- been candid with the American people? I think that's important."

"They certainly know me," McCain added, in reference to the American public.

Obama has rejected such a premise.

"The notion that people don't know who I am is a little hard to swallow," Obama told Gibson on Wednesday, "I've been running for president for the last two years. I've campaigned in 49 states. Millions of people have heard me speak at length on every topic under the sun. I've been involved now in 25 debates, going on my 26th. And I've written two books which any -- everybody who reads them will say are about as honest a set of reflections by, at least, a politician as are out there."

It's the Economy, Again

Obama suggested the McCain campaign's turn toward personal attacks is an effort to avoid discussing the ailing economy.

"Sen. McCain's campaign has been focusing on me primarily because they don't want to focus on the economy. And they've said as much. I mean, you've had their spokespeople over the last couple of days say if we talk about the economic crisis, we lose," Obama said.

But the economy was a top priority for McCain as world markets continue to struggle despite passage of a historic, $700 billion economic rescue package by Congress.

"We are in a crisis of unprecedented proportions. And that's why I recommended strongly that we go out and have the Treasury buy up these bad mortgages, so and arrange it as they did during the Depression," McCain said, referring to a plan he proposed during the second presidential debate in Nashville on Tuesday.

"Sen. Clinton has recommended this," McCain said. "Buy up these mortgages. Let people have them in affordable payment levels. And put a floor on this continuing plummeting of housing -- of home prices. As long as that value continues down, I don't see a stabilization."

But McCain's plan has come under fire from both conservatives and the Obama campaign as one that rewards banks that knowingly provided the riskiest mortgages and bailing out irresponsible homeowners with the taxpayer's money.

"This is a crisis of unprecedented proportions. In many ways it's the same situation we faced in the Great Depression. . . . Right now should we be bailing out all these institutions? Or should we be helping the homeowner in America? We may have to do both, but I'd like to put the homeowner first," McCain said.

McCain's plan would cost an estimated $300 billion -- money, the Republican contender says that could be "part of the $700 billion [economic rescue package], [or] new money, if necessary."

Clinton spokeswoman Kathleen Strand reacted to word McCain had brought up the Clinton proposal in the interview, saying it was offensive to suggest his plan was the same as the Clinton plan. "In the debate, John McCain took credit for coming up with a idea about mortgages, calling it his own proposal. It's offensive to suggest the new McCain plan is what Senator Clinton proposed. She and Senator Obama are focused on helping struggling homeowners and holding bank lenders responsible. The Bush-McCain strategy to sit back and do nothing is wreaking havoc on our economy. Senator McCain's new plan calls for bailing out and rewarding irresponsible bank and mortgage lenders, while sticking taxpayers with the bill," Strand said in a statement.

Gibson asked how McCain would explain to voters why the country needs to buy up these mortgages as opposed to helping those struggling to pay their existing home mortgage.

"Because if your neighbor's home value continues to decline, so is yours. Your life savings -- and I appreciate that you were able to make your payments and struggles that you had. But if your neighbor's home continues to plummet in value and sits empty, then it's going to affect your home as well. So we're doing this for you as well. And hopefully we reach a floor on the home values, and yours and your neighbor's homes can increase in value," McCain said.

McCain Calls for Calm Amid Historic Economic Crisis

McCain defended the need for swift action on the economy but had sharp words for those taking credit for the the economic rescue plan, which he referred to as a "bailout."

"Because of the corruption and cronyism in Washington, no one acted. And it's fascinating to me that the same people that are taking credit for the bailout, the Democrats, [Sen.] Chris Dodd, [Rep.] Barney Frank and others, were defending Fannie and Freddie and the corrupt practices they were doing. I'll tell you. Life's funny," McCain said.

McCain called Freddie Mac and Fannie Mae -- mortgage lenders now owned by the government -- "the catalyst for this catastrophe," but said now is the time for measured leadership.

"I've never been accused of a lack of passion," McCain told Gibson in their exclusive interview, "But the point is [the American people] also want firm, solid, measured, mature leadership. Franklin Delano Roosevelt didn't go on the radio and start yelling. He assured the American people. Right now the American people need seasoned and experienced leadership. That's what I offer."

Gibson challenged McCain on whether either candidate has been able to effectively sell their economic message in a clear, concise way.

"Americans' faith and confidence is shaken," McCain said. "I think I saw a poll where five-percent of the American people think that they are -- that the country's on the right track. They must be the short sellers. I haven't met anyone else. Look, the American people are -- I don't like to use the word fear because Americans by nature know we can overcome any challenge. But they're very badly shaken out there."

"And Senator Obama wants to raise -- increase spending by $860 billion," McCain continued. "He wants to increase the taxes on 50 percent of small business income. He's been all over the place on all of his tax proposals. I have the experience and the knowledge and the judgment to make the right decisions, including on reigning in the growth of government."

The Arizona senator would not comment on whether his campaign had been hurting by weeks of bad economic news.

"I don't know, Charlie. I think by offering wise solutions and providing mature leadership that it'll redound in my behalf," he said.

But there is no question Obama's growth in national polls as well as in key battleground states has coincided with the economic crisis.

"I'm not going to complain about the hand I'm dealt, my friend. We're coming up with solutions. We're trying to give the vision and leadership for the American people. And I'll accept their judgment," he said.