Republican presidential candidate John McCain continued his attack on Barack Obama's credibility Monday, calling his Democratic opponent a liar.

Republican presidential candidate John McCain participates in a rally with his wife Cindy in Albuquerque, N.M. (Gerald Herbert/Associated Press)

McCain told a rally in Albuquerque, N. M., that Obama had accused him of opposing regulation that could have avoided the U.S. financial crisis that has quickly spread to much of the world.

"I guess he believes if a lie is big enough and repeated often enough, it will be believed," he said.

In some of his harshest language yet, McCain said the presidential campaign comes down to a simple question: Who is the real Barack Obama?

McCain drew loud cheers when he said Obama has written two memoirs but "he's not exactly an open book."

Trailing in opinion polls, McCain and his advisers say they'll hammer away at that theme as the campaign heads toward the Nov. 4 election.

His vice-presidential running mate, Sarah Palin, also took shots at Obama over the weekend, questioning his character.

McCain and Obama are set to face off Tuesday in the second of three presidential debates. The event at Belmont University in Nashville, Tenn., will be a town hall format with questions posed by the audience and over the internet, as well as by moderator Tom Brokaw of NBC News.

With files from the Associated Press