On Monday, Cotton organized and drafted an open letter to Iranian leadership -- signed by 46 other Republican senators -- warning them about signing any kind of nuclear deal with the Obama administration.
The White House and others have blasted the letter in recent days as an attempt to undercut the president. The Wall Street Journal published a critical editorial on Monday, calling the letter a "distraction." Vice President Joe Biden condemned Cotton's actions as "beneath the dignity of an institution I revere."
Kelly pushed the Senator even further on Tuesday, questioning how much of an impact his letter even had.
"What’s the point in writing to the Iranian mullahs?" she asked. "They already dismissed it, like, 'Whatever.'"
The Fox News host also suggested to the senator that he may have just botched any opportunity the Republicans had to gain support from the Democratic side.
"You've offended the Obama administration and you may have offended some of the Democrats who would have come over with the Republicans depending on what happens with this deal, to have a stronger say in the Senate,” Kelly said.
But Cotton continued to insist that the letter was "incredibly important" in raising awareness of the "bad deal" President Barack Obama is preparing to make.
"Any deal that's not approved by Congress, won't be accepted by Congress," he said. "Not now and certainly not in the future."