John McCain said Thursday he thinks Barack Obama is trustworthy, even though he has spent the last two weeks portraying his Democratic rival as somebody who can't keep his word.

The presumptive Republican presidential nominee, in an interview with FOX News, seemed to undercut his own message, which has fluctuated since Obama clinched the nomination June 3.

The campaign has hurled several labels at the Illinois senator -- but ever since Obama opted out of public financing for the general election, reneging on a previous pledge to take the money, McCain's campaign has consistently tried to label him as untrustworthy.

Asked if he personally trusts Obama Thursday, McCain said: "I'm sure he's trustworthy. I admire Senator Obama ... I will run a respectful campaign with him."

He said he just doesn't trust him when it comes to campaign finance.

"He broke his word ... that's just a fact," he said.

McCain had stronger words on Saturday during a fundraiser in Kentucky, where he said: "You know, this election is about trust, and trusting people's word ... And unfortunately, apparently on several items, Senator Obama's word cannot be trusted."

And on the plane to Colombia Tuesday night, McCain said, "I think Americans want a leader they can trust and have confidence in and I believe that they will more and more see where Senator Obama has switched his positions on fundamental issues."

He also said, "For him to switch and dramatically change his position -- like public financing, like so many other issues . It's a matter of trust. You have got to take people's word."

McCain spoke with FOX News Thursday in Mexico City, before visiting with Mexican officials and meeting with President Felipe Calderon to discuss illegal immigration. He was in Colombia Tuesday and Wednesday, where he discussed free trade and drug interdiction with President Alvaro Uribe.

He told FOX News it was just a coincidence that he was in Colombia when 15 hostages, including three Americans, were rescued from leftist rebels Wednesday.

"An operation such as this requires weeks or months of planning," he said. "So of course there is no connection between that ... it's just impossible."

He said that in Mexico, Calderon is "truly dedicated" to addressing illegal immigration, and that it will have the "highest priority" in a McCain administration.

Responding to many recent polls that show him trailing Obama, the Arizona senator also said he's comfortable playing the role of the underdog.

"I think I'll be the underdog right up until a minute before the polls close in California," he said. "I think, though, I do best as the underdog."

He spoke freely about the campaign and Obama, even though he said Tuesday such discussions would end at the "water's edge."

McCain said the recent move to put adviser Steve Schmidt in charge of day-to-day campaign operations was just part of a "very great expansion in our campaign."

It has been described as a shakeup, but McCain said Schmidt has gradually taken over more responsibilities. He said Rick Davis remains campaign manager and has "overall responsibility."

"All of us thought, including Rick, that it would be better to give Steve some ... more responsibilities, as we've given other people more responsibilities as the campaign has expanded."

As for his strategy going forward: "We need to keep doing what we're doing, only do it a lot harder."

FOX News' Carl Cameron contributed to this report.