Sen. Marco RubioMarco Antonio RubioGOP senators plan to tune out impeachment week Republicans warn election results are 'wake-up call' for Trump Paul's demand to out whistleblower rankles GOP colleagues MORE (R-Fla.) is questioning Donald TrumpDonald John TrumpGOP senators balk at lengthy impeachment trial Warren goes local in race to build 2020 movement 2020 Democrats make play for veterans' votes MORE's foreign policy chops after the GOP presidential front-runner appeared to stumble in a recent interview.

“If you don’t know the answer to these questions, then you are not going to be able to serve as commander in chief,” Rubio said during a stop in San Juan, Puerto Rico, according to CNN.

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“This should be part of the reason why you are running, because you understand the threats that the world is facing, you have deep understanding and you understand what to do about it,” he said without naming Trump.

“And if someone doesn’t, I think it is very concerning,” added Rubio, himself a 2016 candidate for president.

Rubio’s remarks follow an uncomfortable exchange between Trump and radio host Hugh Hewitt over international affairs earlier this week.

Trump accused Hewitt of pestering him on Thursday night with “gotcha” questions made for embarrassing him over matters of foreign policy.

Rubio said Friday that the outspoken billionaire’s criticism of Hewitt was invalid, given the responsibilities of the presidency.

“No, I don’t,” he responded when asked if Hewitt’s questioning was unfair. “National security is the most important obligation of the federal government.

“If you are going to be a presidential candidate, you need to take this seriously,” Rubio added. “It takes time and dedication and a natural curiosity and interest in these issues.  And I think that’s important.”

Trump initially snapped at Hewitt on Thursday evening after the broadcaster quizzed him over Iran’s Quds Force and Trump responded by discussing Iraq's Kurds.

“I think it’s somewhat ridiculous, but that’s OK, go ahead,” he told Hewitt after admitting he had mistaken “Quds” for “Kurds.”

Trump then insulted the conservative radio host during an interview Friday morning.

“[He is] a third-rate radio announcer,” Trump said on MSNBC’s “Morning Joe.”

“Every question was, ‘Do you know this one or know that one?' ” he added.

Rubio is not alone in his criticism of the real estate mogul’s mistake.

Former Hewlett-Packard CEO Carly Fiorina said Hewitt's questions were appropriate for 2016 contenders such as her and Trump.