ALBUQUERQUE, N.M. - Behind in the polls, Republican John McCain on Monday called Democratic rival Barack Obama a liar as he leveled his harshest criticism yet, and said the campaign boils down to one basic question: Who is Obama really?

Adopting an aggressive tone on the eve of their second debate of the season, the Republican presidential candidate criticized Obama's ties to Chicago, his legislative record and even his pair of best-selling memoirs.

McCain, speaking about the financial crisis, took offense at Obama's accusation that McCain opposed regulation that would have prevented the credit crunch. "I guess he believes if a lie is big enough and repeated often enough it will be believed," McCain said.

The Arizona senator, a veteran of more than two decades in Congress, told his audience that while he is a known quantity the same cannot be said about Obama, who is midway through his first term as a senator from Illinois.

"You need to know who you're putting in the White House - where the candidate came from and what he or she believes," McCain said. "And you need to know now, before it is time to choose."

Later, he added: "There are essential things that we don't know about Sen. Obama or the record he brings to this campaign."

Obama campaign spokesman Tommy Vietor said McCain is a "truly angry candidate" who is trying to divert attention from the economy and that it was Obama who warned, in 2007, of the subprime mortgage crisis now blamed for the turmoil in the financial industry. Vietor said McCain has been consistent in calling for less regulation, "proving that he hasn't learned any lessons from the last banking scandal he was involved in."

That was a reference to Charles Keating, a savings and loan financier and McCain friend and campaign contributor who ultimately was convicted of securities fraud. Just months into his Senate career in the late 1980s, McCain made what he has called "the worst mistake of my life" by participating in meetings with banking regulators on behalf of Keating.

The Senate ethics committee investigated five senators' relationships with Keating; McCain was cited for a lesser role than the others, including his "poor judgment."

McCain and his advisers plan to hammer the theme that Obama is an untested candidate who has not faced legitimate scrutiny as they try to close the gap in the final four weeks before Election Day, Nov. 4.

The Arizona senator spent the weekend working with his advisers to sharpen a line of attack against Obama, who has jumped ahead of the Republican in several critical swing states. Aides said the tinge of uncertainty about Obama is their way to puncture his lead in the polls.