Senators Rand Paul and Marco Rubio are sparring over Cuba, in what may be the first overt philosophical debate between two likely Republican presidential candidates in the run-up to the 2016 primaries.

Rubio threw the first punch, telling Megyn Kelly on Fox News that Paul “has no idea what he’s talking about.” Paul, meanwhile, has taken to cyberbullying Rubio, tweeting on Friday that he is “acting like an isolationist.”

Almost as soon as the words “through a policy of engagement, we can more effectively stand up for our values and help the Cuban people help themselves” left President Obama’s mouth on Wednesday, Rubio was out in front of his own podium, colorfully denouncing the idea.

Where there was a camera, there was Florida’s junior U.S. senator, whose parents fled pre-Castro Cuba, lambasting the policy as everything from “outrageous and ridiculous” to a “concession to a tyranny.”

Paul took a different approach: he slept on it. Reporters who sought comment from the senator’s office on Wednesday were mostly given no comments, or cloudy statements that sort of sounded like approval for lifting the embargo, but didn’t quite go as far as to say it. “In the past, Senator Paul has stated that he believes in more trade, not less, and that includes Cuba. Peace through commerce is one of Paul’s firm beliefs,” a Paul spokesperson told The Daily Beast at the time.

By Thursday morning, Paul was prepared to make his position clear.

“The 50-year embargo just hasn’t worked,” he said on West Virginia’s News Talk 800. “If the goal is regime change, it sure doesn’t seem to be working, and probably it punishes the people more than the regime, because the regime can blame the embargo for the hardship.”

Siding with Obama on anything, no matter how sensical, is a risky move for a would-be Republican presidential candidate. But Paul’s position is in sync with the libertarian strain of the GOP, and with that of his father, former congressman and presidential candidate Ron Paul, who is something of a libertarian icon.

“The conservative approach should be that free markets are the best weapon again[st] communism, not isolationism,” Jack Hunter, a former Paul staffer and currently the editor of Rare, a libertarian website, wrote on Facebook of Paul’s position.

More mainstream Republicans, Rubio chief among them, disagreed.

On Fox News Thursday evening, Rubio was asked what he thought of “Senator Rand Paul’s comments supporting the president.” He teed off on Paul.

“Like many people, he has no idea what he’s talking about…I would expect that people would understand that if they just took a moment to analyze that, they would realize that the embargo is not what’s hurting the Cuban people. It’s the lack of freedom and the lack of competent leaders.”

Paul responded first on Facebook, where he, in part, accused Rubio of “want[ing] to continue perpetuating failed policies,” and of “acting like an isolationist who wants to retreat to our borders and perhaps build a moat. I reject this isolationism.”

Then, Paul took to Twitter to reiterate his point, reusing the isolationist-moat line, which I’m sure his team deems particularly clever seeing has Paul’s foreign policy is often dismissed by more mainstream Republicans as being “isolationist” due to his past proposals to get rid of all foreign aid, and his statements that military intervention exacerbates chaos in the Middle East—a reputation he has spent the last year trying to shake.

Lest his attacks be waved off has lacking substance, Paul quickly scurried over to Time, where he elaborated on his anti-isolationist Cuba platform without mentioning Rubio by name once.

But as Bloomberg Politics’ Dave Weigel noted, Paul’s decision to come out swinging against Rubio on this particular issue was no small decision.

"Paul had attacked him on a strength, on the issue Rubio is most comfortable talking about—and the one that leaves him most prone to vehemence and emotion.”

Not that it was necessarily a surprise.

When it comes to debates, Paul approaches a challenge with gleeful menace, as it allows him to employ his I’m-smarter-than-all-of-you attitude that his supporters seem to love. Paul has played the role of bullfighter more than any other Republican presidential prospect, in the past waving a muleta at New Jersey Governor Chris Christie over his “bully demeanor,” and Texas Governor Rick Perry over foreign policy (Neither governors could be baited into this battle, however, since they have both stayed mum on Cuba.)