Connecticut Sen. Chris Murphy warned that a decision by Congress not to approve the Iran nuclear deal in the next two months would significantly undermine Obama's presidential legitimacy, and the legitimacy of all presidents who follow him.

"This would be an absolute blow to the legitimacy of this president, and of any president to negotiate a diplomatic agreement," the Democrat said Thursday on MSNBC.

He added: "This is an exceptional moment where you have the United States, our European partners, Russia and China all agreeing on a path forward, and if the United States Congress was to override that, I don't know how any president in the future can ever sit across our allies and adversaries and negotiate a deal."

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Murphy, who sits on the Senate Committee on Foreign Relations, criticized Republicans for a "casualness of opposition" despite what would happen should Congress reject the Iran deal.

"Republicans in Congress just simply don't believe in the legitimacy of diplomacy as a tool in the toolkit of the American president," he said. "They do zero oversight on our war we are fighting right now against [the Islamic State], military engagement in the Middle East, but they do oodles of oversight on a diplomatic agreement with Iran."

The deal, reached this week between Iran, the U.S., Germany, France, Britain, Russia and China, curbs Tehran's nuclear ambitions in exchange for sanctions relief. Once it's delivered to Congress, which should happen in the coming days, Congress will have 60 days to either approve or disapprove of the deal, or take no action, in which case the deal will be automatically approved.