By Geneva Sands - 01/13/12 03:06 PM EST
McCain, who was the GOP nominee in 2008, told late-night host David Letterman on Thursday that the primary process is a "very high-pressure situation."
"It's very difficult, and it should be terribly difficult. You're talking about the highest office in the world. And so, sometimes people say things that in fact I, on occasion, have said not-so-smart things from ... I won't remind you," he said on the "Late Show With David Letterman."
Meanwhile, Letterman asked McCain if he was in touch with his former running mate, Sarah Palin.
"In fact, I talked to her a couple days ago, and she's doing very well," McCain said.
"What is she doing?" asked Letterman.
"She's a commentator on Fox. She's been involved in some of the campaigns. You and I have had this conversation several times in the past. I am very well-aware of your view on this issue," said McCain, laughing.
McCain also weighed in on the current GOP contenders, saying he believes Mitt Romney will win the Republican nomination.
"I think he deserves it," said McCain.
McCain endorsed the former Massachusetts governor last week and has been stumping for him in New Hampshire and South Carolina.
When asked about GOP hopeful Rick Santorum, McCain said he had respect for the former Pennsylvania senator, but disagreed with him over earmarks.
"I thought, as a number of people have said, that these earmarks are a gateway drug to corruption. We just did not agree in those days when he was in the Senate before he was beaten," McCain said.
Santorum lost his Senate seat to Democratic opponent Bob Casey Jr. in 2007.
A pro-Romney group adopted a similar line of attack, releasing a Florida TV ad Friday that blasts Santorum over his support of earmarks in Congress and paints him as unelectable against President Obama.