Rand Paul is hammering Marco Rubio over Cuba on Twitter. Welcome to the 2016 primary.

Thursday morning, Sen. Rand Paul came out in favor of President Obama’s new plan to normalize relations with Cuba, saying in a radio interview that “the 50-year embargo just hasn’t worked” and that “opening up Cuba is probably a good idea.” Later that day, Sen. Marco Rubio — a potential 2016 presidential rival to Paul who’s called the deal “disgraceful“ and a ”concession to tyranny“ — said on Fox News that Paul ”has no idea what he’s talking about.”

So early Friday afternoon, Paul fired back, and directly challenged Rubio with a series of confrontational Twitter and Facebook posts. Here’s his short tweetstorm:

The “isolationist” jab is Paul’s sarcastic attempt to repurpose an epithet that’s frequently applied to him. The metaphorical reference to a “moat” is a bit strange, though, given that there already is a larger-than-moat-sized water barrier between the US and Cuba.

At press time, Sen. Rubio had not posted a social media response. But if Paul runs for president — as he looks highly likely to — it’s clearer than ever that the GOP primary will feature a spirited debate over foreign policy.