Sen. Marco RubioMarco RubioRepublicans giving Univision the cold shoulder: report Week ahead: Senate panel to vote on Trump's Labor pick Senators introduce new Iran sanctions MORE (Fla.) slammed his colleague Sen. Rand PaulRand PaulTrump: 'No doubt' we'll make a deal on healthcare Overnight Defense: General says US strike probably led to civilian deaths | Tillerson to push NATO on spending | Trump taps F-35 chief Senate backs Montenegro's NATO membership MORE (Ky.) as a “committed isolationist” as the pair sparred over taxes in the fourth GOP presidential debate Tuesday.

“Is it conservative to have a trillion dollars in transfer payments, a new welfare program, that's a refundable tax credit?” Paul said of Rubio's tax plan.

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“Add that to Marco's plan for $1 trillion in new military spending and you get something that looks to me not very conservative,” Paul said. 

Rubio, who has been rising toward the top of the crowded GOP field, defended his tax plan before pivoting to a desire to “rebuild the American military.”

“I know that Rand is a committed isolationist. I am not,” Rubio said. “I know the world is a safer and better place when America is the strongest military power in the world.”

Paul, the most libertarian-leaning candidate in the 15-person GOP field, has maintained a non-interventionist position on foreign policy, an issue where Rubio is among the most hawkish.

“Marco, Marco, how is it conservative ... to add a trillion dollars in military expenditures?” Paul asked. 

“You cannot be a conservative if you're going to keep promoting new programs that you're not going to pay for,” he added to applause. 

“We cannot even have an economy if we're not safe,” Rubio responded, mentioning threats from Islamic militants and Chinese aggression in the South China Sea. 

“The world is a stronger and better place when the United States is the strongest military power in the world,” Rubio said to roaring applause. 

Paul did not relent on the issue, interjecting, “I do not think we are any safer from bankruptcy court. As we go further and further into debt, we become less and less safe.”