He hasn't been as outspoken as some GOP contenders in condemning Donald Trump. He’s been reluctant to criticize other Republicans, and he was off the campaign trail this past week getting over a cold.
"The Iranian regime and the world should know that a majority of members in this Congress do not support this deal and that the deal could go away on the day President Obama leaves office," a noticeably angry Rubio told Secretary of State John Kerry on Thursday.
Rubio added: "The Iranian regime and the world should know this deal -- this deal is your deal with Iran.”
That same day, in one of three national television interviews since the hearing, Rubio made it clear that as president he would "absolutely" re-impose sanctions on Iran.
"What the President is using is a national security waiver. He’s in essence saying, ‘I’m using this waiver to prevent the sanctions from still being imposed’,” Rubio said in an interview on Fox News. "The next President could just lift that with the stroke of a pen."
Although many other Republicans candidates have also been critical of the deal to keep Iran’s nuclear ambitions in check, the issue seems to send Rubio fuming. On Wednesday, Rubio said President Obama had “no class” for going “on comedy shows to talk about something as serious as Iran”, referring to the President’s interview on the Daily Show with Jon Stewart.
As a member of the Senate Committee on Foreign Relations, Rubio has made his opposition to the deal a pillar of his campaign, emerging as one of the its harshest and most outspoken critics.
One of the outside groups backing his presidential bid, the Conservative Solutions Project, has already released two ads criticizing the deal, calling it a "bad deal" and claiming it would give Iran "a clear path to a nuclear bomb."
At the Senate hearing, Kerry told Rubio he was confident the next president would have enough common sense to not arbitrarily end the deal if it was being properly implemented.
“If you think the Ayatollah is going to come back and negotiate again with an American, that’s fantasy,” Kerry said. “You’re never going to see that because we will have proven we’re not trustworthy."
"The fantasy is in believing that they’re going to even live to the accord," Rubio shot back in an interview with CBS on Friday. "This is a country that has a long history, at least the leaders of Iran do, of violating agreements and of always having a secret nuclear program."