Sen. John McCain (R-AZ) didn't provide the most ringing endorsement of Mitt Romney's Tuesday morning, claiming the GOP presidential frontrunner has underperformed on the campaign trail thus far.

In a phone interview with Fox Business Network's Don Imus, McCain expressed confidence in Romney still winning the nomination before mentioning his struggles.

“Some of the pundits I was watching this morning – ‘Well, he may not be able to get there if he doesn’t do well in Wisconsin, blah, blah, blah,’" he said. "I think he has got a solid lead. I think he has not done as well as we had hoped. None of us ever do in campaigns."

When Imus called Romney a "terrible candidate," McCain quickly came to the former Massachusetts governor's defense.

"Oh, that’s not true, Don,” he said.

WATCH: Video from Fox Business Network, which was broadcast on March 20, 2012.