Sen. Rand Paul (R-Kentucky), is introduced to members of the Londonderry Fish and Game Club, Wednesday, Jan. 14, 2015, in Litchfield, N.H. AP/Jim Cole Sen. Rand Paul (R-Kentucky) is emerging as the most aggressive candidate in the 2016 presidential race.

While the other likely hopefuls have largely avoided taking direct shots at each other, the libertarian-oriented senator has taken swipe after swipe at his potential foes. Most recently, he suggested another Mitt Romney candidacy would be the "definition of insanity."

"When you do the same thing and expect a different result, it’s sort of what Einstein said, that the definition of insanity is to do the same thing over and over again and expect a different result," Paul said of Romney on Tuesday, according to the New Hampshire Journal.

But that was hardly Paul's only dig at his rivals, as The Washington Post noted earlier in the week. In a January interview with Breitbart News, Paul tore into both New Jersey Gov. Chris Christie (R) and former Florida Gov. Jeb Bush (R) for their policy positions. Christie, he argued, was too far to the left on gun control issues

"I just think it’ll be difficult for anybody to win the nomination who’s not a staunch defender of the Second Amendment," Paul said of Christie.

And Paul contended Bush's support for Common Core educational standards would place him out of step with GOP primary voters. Paul said a "moderate" like Bush would still have a place in the Republican Party — just not as its presidential nominee.

"When we have a primary, voters will have to pick whether they want a moderate leading the party or a conservative," Paul said while discussing Bush. "That's what the primary will be about, people presenting their ideas and they'll have to decide whether they want Common Core, whether they want more spending, more taxes, whether they want a candidate who will not pledge to not raise taxes. There's a lot of things that will go on, we have plenty of time for that, but I would say that time will tell."

Perhaps Paul's most aggressive attacks have been against Texas Gov. Rick Perry (R), who wrote a Washington Post op-ed criticizing Paul's "isolationist" foreign policy. Paul responded with an op-ed of his own in Politico.

"There are obviously many important events going on in the world right now, but with 60,000 foreign children streaming across the Texas border, I am surprised Governor Perry has apparently still found time to mischaracterize and attack my foreign policy," Paul wrote.

This was not the only time Paul had taken out an op-ed to criticize a potential presidential candidate from the Lone Star State. The day after Sen. Ted Cruz (R-Texas) invoked President Ronald Reagan's name to differentiate himself with Paul on foreign policy, Paul wrote a Breitbart op-ed taking unnamed Republicans to task for misrepresenting Reagan's legacy.

"What we don’t need right now is politicians who have never seen war talking tough for the sake of their political careers. America deserves better than that. So do our soldiers," Paul declared.

Paul has also directly gone after Sen. Marco Rubio (R-Florida) on foreign policy. At the end of last year, President Barack Obama moved to normalize relations with Cuba, which Paul supported. Rubio attacked the "Obama-Paul" policy on Cuba in response and Paul, obviously, did not ignore Rubio either:

—Senator Rand Paul (@SenRandPaul) December 21, 2014

—Senator Rand Paul (@SenRandPaul) December 19, 2014

—Senator Rand Paul (@SenRandPaul) December 19, 2014

—Senator Rand Paul (@SenRandPaul) December 19, 2014

—Senator Rand Paul (@SenRandPaul) December 19, 2014