Sen. John McCain (R-Ariz.) openly disagreed with House Majority Leader John Boehner (R-Ohio) on Tuesday morning, saying he believed there was no problem with criticizing the president of the United States while he is overseas on official business.

The controversy arose when Mitt Romney went after President Barack Obama for signaling to Russian President Dmitry Medvedev that he would have more "flexibility" to negotiate on missile defense after the November elections.

"Russia is not a friendly character on the world stage, and for this president to be looking for greater flexibility, where he doesn't have to answer to the American people in his relations with Russia is very, very troubling, very alarming," Romney said. "I'm very, very concerned."

The former Massachusetts governor's opinion prompted a response from Boehner, who said it was inappropriate to criticize the president on foreign affairs while he was abroad.

"Clearly while the president is overseas, he's at a conference and while the president is overseas I think it's appropriate that people not be critical of him or our country," Boehner said when asked by NBC News whether he agreed with Romney that Russia is the "number one geopolitical foe" of the United States.

But on Fox News Tuesday morning, McCain took the side of Romney, whom he is backing in the GOP primary:

Well, I understand John Boehner's point and I respect that. But this is a very serious issue. This is very serious. no matter where the president is. If he makes a statement that I think could endanger the United States' national interest, I have to respond no matter where the president of the United States is. I spent too many years in efforts working with Republican and Democrat administrations to try to protect this nation and missile defense -- and it is a defensive system, not an offensive system. Why would the Russians object so much to a defensive system?

All I can say is that I respectfully disagree with Speaker Boehner. When something that important is said by the president of the United States of America, we have to respond.

When host Brian Kilmeade then asked McCain if he agrees with Romney that Russia is America's "number one geopolitical foe," the senator responded, "I think in many respects. Look at what they're doing in Syria. They're supplying arms and equipment to Bashar Assad while he slaughters and massacres his own people. Look at what -- they continue to prop up North Korea. They continue to -- and obviously now there is now a president for life."

WATCH: