McCain touted Romney as the "most qualified" of the GOP candidates, and said Romney similarly put aside any prior attacks to back McCain in the last general election.

"After the 2008 [primary] campaign was over, no one worked harder on behalf of my campaign than Mitt Romney did," he said. "You put these things aside ... after it's over, you share the same principles."

McCain added that he would back whichever candidate Republican primary voters elect to represent them against President Obama.