Republican John McCain is calling Democratic rival Barack Obama a liar.

The GOP presidential candidate told a campaign rally: “Sen. Obama has accused me of opposing regulation to avert this crisis. I guess he believes if a lie is big enough and repeated often enough it will be believed.”

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

McCain drew the loudest cheers when he said the Democrat has written two memoirs but “he’s not exactly an open book.”

Trailing in the polls, McCain and his advisers say they will hammer that theme as the campaign heads toward the Nov. 4 election.