Freshman U.S. Sen. Tom Cotton, R-Ark., Sunday stuck by his controversial letter to Iranian leaders and critized Tehran's reaction to it, saying Iranian leaders just don't understand how the "constitutional system works." In a separate interview, Senate Majority Leader Mitch McConnell called Democratic criticism of the letter signed by 47 Republican senators "a manufactured controversy."

Cotton penned a letter warning Iranian leaders any deal worked out to stem its nuclear program with the Obama administration could be overturned once Obama leaves office in 2016. Obama has said he would veto any bill that gives Congress final say over a deal since the U.S. Constitution provides for the executive branch to conduct foreign policy with the "advice and consent" of the Senate.

Negotiations between Iran and the P5+1 nations -- the five permanent members of the U.N. Security Council plus Germany -- on Tehran's nuclear program are nearing a March 24 deadline for a framework deal, with a final agreement due in June. Both President Obama and Secretary of State John Kerry have said the talks are at a critical stage and accused Cotton and his 46 colleagues of trying to sabotage an agreement.

Cotton, appearing on CBS's "Face the Nation," said he has "no regrets at all" about sending the letter.

"It's a simple fact of our Constitution that if Congress does not approve that deal, then it may not last," Cotton said, adding Congress is ready to impose even more sanctions on Iran, which contends its nuclear program is for peaceful purposes only.

Iranian Foreign Minister Mohamed Zarif dismissed the letter as propaganda, but Cotton characterized the negotiations as a back-door deal and "not the way our constitutional system works."

"He thinks international law can override our Constitution," Cotton said. "Congress has to approve them for them to be lasting and binding."

On CNN's "State of the Union," McConnell warned Obama "is about to make what we believe is a very bad deal. He clearly doesn't want Congress involved at all. And we're worried about it."

Kerry, on "Face the Nation," called the Republican interference "unprecendented" and urged Congress to wait until the deal is concluded before judging it. He also said he won't apologize to the Iranians for the letter.

"I'm not going to apologize for the unconstitutional, un-thought-out action by somebody who's been in the United States Senate for 60-something days," said Kerry, who is in Geneva for a final round of talks on the framework. "That's just inappropriate.

"The effect and the intent of the author was to basically say, 'Don't do this deal,'" Kerry said. "It's like -- you know -- giving people a grade on a test before the test is even written, let alone given. It's wrong, it's unprecedented, and I hope it hasn't made it very difficult here."

White House Chief of Staff Denis McDonough said late Saturday in a letter to Senate Foreign Relations Committee Chairman Bob Corker, R-Tenn., congressional efforts to wrest control of the negotiations from the White House "would have a profoundly negative impact" that would embolden Iranian hardliners. "The legislation would potentially prevent any deal from succeeding by suggesting that Congress must vote to 'approve' any deal and by removing existing sanctions waiver authorities that have already been granted to the president," McDonough wrote.