WASHINGTON >> Secretary of State John Kerry, returning to talks with Iran on its nuclear program, said Sunday that most of the differences still barring an agreement are political rather than technical.

Kerry was to meet in Switzerland with Iranian Foreign Minister Mohammed Javad Zarif. The two sides are working to get a deal by the end of March.

“Most of the differences now are political decisions that need to be made in order to fulfill the promise of proving to the world that a program is peaceful,” he said in an interview on CBS News.

Kerry said that Tehran “to its credit has thus far lived up to every part of the agreement we made over a year ago.” He said it’s unclear at this stage whether a letter from 47 GOP senators has jeopardized talks aimed at curbing Iran’s nuclear program. However, he said he will make clear that Congress does not have the right to change an executive agreement between nations.

Senate Majority Leader Mitch McConnell defended the letter from the 47 senators. Its critics have a case of selective outrage, he said, citing past instances in which he maintained that Democratic lawmakers had attempted to undercut the foreign policy priorities of the Republican Reagan administration.

“The main point here I think everybody needs to understand is that the president is about to make what we believe will be a very bad deal,” McConnell said Sunday on CNN. “He clearly doesn’t want Congress involved in it at all, and we’re worried about it.”

Some Republican senators up for re-election next year have experienced heavy criticism back in their home states for signing the letter. McConnell said he wasn’t worried about the backlash.

The author of the controversial letter, Sen. Tom Cotton, R-Ark., said Sunday he had no regrets.

“The fact that President Obama doesn’t see this letter as a way to get more leverage at the negotiating table just underscores that he is not negotiating for the hardest deal possible,” Cotton said on CBS’s “Face the Nation.”

Kerry’s interview was aired a day after White House chief of staff Denis McDonough sent a letter to Senate Foreign Relations Committee Chairman Bob Corker warning Congress once more that it should not interfere in the negotiating process. Corker has expressed concerns last week in a letter separate from the 47 senators’ about Congress being excluded from the process.

In his letter to Corker, McDonough said, “The administration’s request to Congress is simple: Let us complete the negotiations before the Congress acts on legislation.” He added that he does expect a robust congressional debate if a final deal is struck by the end of June.

And McDonough reiterated Obama’s repeated threats to veto the legislation should Congress pass it.

Corker and Senate colleagues in both parties insist that Congress be allowed to consider and vote on any agreement designed to block Iran from developing a nuclear weapon. Corker argued his case his letter to Obama and again in response to McDonough.

“On this issue where Congress has played such a vital role, I believe it is very important that Congress appropriately weigh in before any final agreement is implemented,” Corker said in a statement late Saturday.

Tensions between the administration and lawmakers over Iran have been rising for weeks.