"I believe much of our strategy with regards to ISIS is being driven by desire not to upset Iran so that they don't walk away from the negotiating table," said Sen. Marco Rubio, R-Fla., referring to the emerging U.S.-led deal to curb Iran's nuclear program. "Tell me why I'm wrong."
"The facts completely contradict that," Kerry responded, adding that "you are misreading that if you think there is not a mutual interest" between the U.S. and Iran to defeat the Islamic State.
Kerry tried to clear up what he sees as a misinterpretation of the Iran negotiations by members of Congress. He said the talks are about preventing Iran from developing a nuclear weapon—no other issues are on the table during negotiations.
"It is almost insulting that the presumption here is that we are going to negotiate something that allows them to get a nuclear weapon," he said.
Sen. Chris Murphy, D-Conn., asked Kerry for his views on the "unprecedented" letter that 47 Republican senators sent to Iran.
"Nobody is questioning anybody's right to dissent. ... But to write and suggest that they are going to give a constitutional lesson—which, by the way, was absolutely incorrect—is quite stunning," Kerry said, noting that the vast majority of international agreements, since the country's founding, have not required the consent of the Senate. The letter had warned Iranian officials that any such deals would have to be approved by Congress. "This letter ignores more than two centuries of precedence in the conduct of American foreign policy."
Senate Foreign Relations Committee Chairman Bob Corker, another Republican who didn't sign the letter, tried to cut in during Kerry's response. "Mr. Secretary, I know that is a well-written speech—" But Kerry, visibly irritated, didn't let him.
"This is not a speech. This is about the impact of that irresponsible letter," Kerry said, later adding, referring to Murphy, "I'm asked by one senator what the impact is, and I am laying out what the impact is."
Sen. Jim Risch, R-Idaho, defended his signing of the Iran letter in response to Kerry's and others' comments about the missive.
"This indignation and beating over this letter is absolute nonsense," Risch said, adding that each senator who signed the letter had the authority to do so as an elected official in the "first branch of this government."
"To say that we should not be communicating is nonsense," Risch said, because members of Congress consistently communicate with foreign leaders.
A day before the hearing, Republican Sen. Lindsey Graham, one of the more hawkish members of his party, told National Journal that Democrats and Republicans are too divided on the issue for the AUMF to go through.
"Oh, the AUMF is dead on arrival ... it won't work," Graham said. "I want to keep it really simple. Do what you need to to destroy and degrade ISIS wherever they are located, period. ...I think the libertarians and the liberals would say 'no.' "