Kelly grilled the freshman Republican senator on whether the letter has been helpful to ongoing U.S.-led negotiations over the country’s nuclear program. Cotton objected when Kelly mentioned a recent editorial published by The Wall Street Journal that called the letter “ a distraction.”

“Well, on the contrary, Megyn, I think that this debate we’re having is incredibly important and helpful to raising just what a bad deal President [Barack] Obama is about to make with Iran,” Cotton said. “The last two days, we’ve focused on the terms of the deal, which is: One, President Obama will accept a 10-year sunset clause; and two, he has conceded a vast Iranian enrichment capability.”

Kelly then asked Cotton what he hoped to accomplish in addressing top Iranian leaders.

“But what’s the point in writing to the Iranian mullahs? What are you gonna do?” she asked. “They dismissed it already like ‘pfft, whatever,’ and 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.”

Cotton responded that Iran experts say the country’s leaders don’t understand the U.S. Constitution.

“So we need to be crystal clear with the leaders of Iran: Any deal that’s not approved by Congress won’t be accepted by Congress,” he said. “Not now, and certainly not in the future, because Congress is focused on stopping Iran from getting a nuclear weapon. Today, tomorrow and 10 years from now.”

The Arkansas senator said he still wants Democrats to join with Republicans on the letter and in their broader efforts to halt, or at least shape, an agreement.

“We’re encouraging the Democrats to join us to protect Congress’ constitutional responsibilities,” he said.

Asked again about Vice President Joe Biden’s statement against the senator’s action, Cotton said the vice president’s “focus on the process just goes to show he knows he can’t defend the deal.”

Cotton also extended an invitation to Biden to debate the issue.

“I would be happy to debate Vice President Biden, one on one, at any time, in a place of his choosing, maybe right here on your show, Megyn,” he said.