Story highlights Sen. John McCain insists Sen. Rand Paul is an isolationist

McCain: Paul has been "doing somersaults" after ISIS beheadings of Westerners

Dick Cheney also has been critical of Paul on foreign policy

Paul, a possible 2016 GOP presidential candidate, denies switching foreign policy views

Sen. Rand Paul is trying to shed the perception that he's an isolationist and that he thinks the United States should retreat from intervention in other countries' affairs. But members of his own party are making it hard for the Kentucky Republican to escape that image.

Sen. John McCain, one of the most hawkish members of the Senate, told Fox News host Neil Cavuto on Tuesday night that Paul has "a fundamental lack of understanding of the situation and the threats we face" from ISIS.

Referring to Paul's evolving position on ISIS, McCain said the libertarian-leaning senator "has obviously been doing somersaults" since the Sunni militant group gruesomely killed two American journalists and a British aid worker.

Paul, who is seriously considering a presidential run in 2016, responded to his critics in an interview released Wednesday in The Federalist , telling the conservative Web magazine how "frustrating" these labels are for him.

"I spent the past five years in public life telling everyone that 'hey, I'm not an isolationist' ... and when they find out I'm not, they say I've switched positions, because I'm not the position they were saying I was. You know what I mean? So for five years they've been accusing me of being something that I say I'm not."

But McCain apparently doesn't buy this explanation. He said Paul has "dramatically shifted his positions on national security. He said we shouldn't intervene, no matter what, anywhere. And now obviously he wants to take out ISIS."

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"I think it's kind of a desperation kind of trying to find some footing here as he slides down a very steep slope of credibility."

Former Vice President Dick Cheney has also been critical of Paul's foreign policy stance. On ABC's "This Week" in June, Cheney described him as "basically an isolationist. That didn't work in the 1930s; it sure as heck won't work in the aftermath of 9/11."

Facing such vocal opposition in the Republican Party, Paul continues to battle the image of an isolationist with shifting views.

Paul told CBS "This Morning" earlier this week that his perception of ISIS has evolved as world events changed, adding that the killings of Steven Sotloff, James Foley and David Haines "influenced" his position.

But Paul has been noncommittal on what the United States should do except that the Congress Paul has said he doesn't back arming Syrian rebels, a position McCain has supported since Syria's civil war broke out three years ago.But Paul has been noncommittal on what the United States should do except that the Congress should determine the path forward.

In The Federalist interview, Paul didn't deny that he's skeptical of U.S. intervention. "We have to be very wary of intervention and that there are often unintended consequences," he said.

"At the same time, I've also said all along that I'm not for no interventions."

McCain, however, remains skeptical.

When Cavuto asked him if Paul would be a good commander in chief, he replied, "Not on national security. No."