Secretary of State John Kerry and Sen. Marco Rubio (R-Fla.) shared a tense exchange about U.S. relations with Iran during a hearing on the president's request for authorization of the use of military force against the Islamic State on Wednesday. (AP)

This post has been updated.

The Wednesday morning hearing held by the Senate Foreign Relations Committee was titled, "The President's Request for Authorization to Use Force Against ISIS: Military and Diplomatic Efforts." And for about an hour, that, for the most part, was the topic discussed.

But when Sen. Marco Rubio (R-Fla.), potential 2016 presidential candidate and leading light of the Republican Party, took the mike, he directed the conversation away from the Islamic State to the dominant foreign policy issue on Capitol Hill in recent weeks and what is quickly developing into the dominant foreign policy issue of the presidential race: America's relations with Iran.

"I believe that much of our strategy with regards to ISIS is being driven by a desire not to upset Iran so that they don't walk away from the negotiating table on the deal that you're working on," Rubio said to Secretary of State John F. Kerry. "Tell me why I'm wrong."

"Because the facts completely contradict that," Kerry replied. "But I'm not at liberty to discuss all of them here for a lot of different reasons."

Rubio was suggesting that the Obama administration is stinting in its airstrikes against the Islamic State in order to allay Iranian anxiety about a new American military incursion into the region. In preamble questions for Defense Secretary Ashton Carter, Rubio suggested that any American military presence would threaten Iran's desire to be a regional "hegemon." And by holding back, Rubio suggests, Obama is paving the way for a diplomatic deal on the Iranian nuclear program -- a deal that Republicans on and off Capitol Hill are deeply skeptical about.

Both Carter and Kerry challenged Rubio's reading, saying that the United States and Iran actually have common cause against the Islamic State -- a Sunni group which Shiite Iran sees as every bit a threat to their regional influence as American bombers.

"They want us to destroy ISIS; they want to destroy ISIS," Kerry said. "ISIS is a threat to them; it's a threat to the region. And I think you're misreading it if you don't think that there's a mutual interest ... between every country in the region." Kerry later challenged Rubio's contention that Sunni regimes in the Middle East, including key allies, such as Saudi Arabia and the United Arab Emirates, are opposing the nuclear negotiations.

Rubio's possible misreading aside, his line of questioning is the latest indication that a hard line against Iran will be a litmus test for ambitious Republicans and the most important foreign policy issue of the developing presidential race.

Rubio, along with fellow 2016ers Ted Cruz (R-Texas) and Rand Paul (R-Ky.), signed the letter this week pointing out to Iranian leaders that a future president could act to undermine any nuclear deal agreed to by Obama and the multinational partners now negotiating with Iran in Switzerland. And all three attended and made approving comments about Israeli Prime Minister Benjamin Netanyahu's address to Congress last week that was deeply critical of a nuclear deal.

Paul, like Rubio a Foreign Relations Committee member, also questioned Kerry and Carter Wednesday. His remarks hewed closer to the Islamic State force authorization -- hammering Obama for, in his view, outstripping his constitutional war powers by launching airstrikes against the group -- but he also highlighted his decision to sign the Cotton letter.

"The letter was to you," Paul told Kerry and Carter. "The letter was to Iran, but it should've been CC’d to the White House, because the White House needs to understand that any agreement that removes or changes legislation will have to be passed by us."

Meanwhile, the Republican 2016 hopefuls off Capitol Hill have been sharpening their rhetoric on Iran in recent weeks as GOP opposition to the nuclear talks has hardened.

Former Texas governor Rick Perry released a video last week lambasting the Obama administration over the Iran talks, while Louisiana Gov. Bobby Jindal, Wisconsin Gov. Scott Walker and former Florida governor Jeb Bush all expressed support for the Republican senators' letter to the Iranian regime.