In an open letter to Iranian leaders, Sen. Tom Cotton, R-Ark., and 46 other Republicans said that without congressional approval, any deal between Iran and the U.S. would be merely an agreement between President Barack Obama and Iran's supreme leader, Ayatollah Ali Khamenei.
"The next president could revoke such an executive agreement with the stroke of a pen," they wrote, "and future Congresses could modify the terms of the agreement at any time."
The U.S. and other nations are seeking a pact that would let Western powers verify that Iran will not obtain a nuclear weapon.
Though the five-paragraph letter is addressed to leaders in Tehran, it seems as much aimed at delivering a message to Obama.
Republicans and some Democrats want Congress to vote on any agreement. The pact the bargainers are working on does not require congressional approval because it is not a treaty, which would require a two-thirds majority Senate vote to be ratified.
Sen. Dick Durbin of Illinois, the No. 2 Democrat in the Senate, harshly criticized the GOP and accused the Republicans of risking another war in the Middle East.
"This is a cynical effort by Republican senators to undermine sensitive international negotiations — it weakens America's hand and highlights our political divisions to the rest of the world," Durbin said in statement. "Understand that if these negotiations fail, a military response to Iran developing their nuclear capability becomes more likely. These Republican senators should think twice about whether their political stunt is worth the threat of another war in the Middle East."
Last week, Senate Majority Leader Mitch McConnell, R-Ky., postponed action on legislation that would give Congress a vote on any deal that emerges. He delayed the measure in the face of solid Democratic opposition to moving ahead on the bill now, just weeks before an end-of-March deadline for negotiators to produce an outline of an agreement.
On CBS' "Face the Nation" Sunday, McConnell said the president would need congressional approval to lift sanctions already imposed on Tehran because of its nuclear program.
"I think he cannot work around Congress forever," McConnell said.
Obama said in a separate CBS interview that the U.S. would "walk away" from the talks unless they produce a procedure for verifying that restrictions on Iran's nuclear effort are working.
The letter released Monday was signed by 47 of the Senate's 54 Republicans. Included were McConnell and the rest of the Senate GOP leadership plus presidential contenders Marco Rubio of Florida, Rand Paul of Kentucky and Ted Cruz of Texas.
Missing from the letter were seven Republicans, including Sen. Bob Corker of Tennessee, the chairman of the Foreign Relations Committee.
The next negotiations are scheduled for March 15, and wide gaps remain between the two sides.
Iran has said its nuclear program is peaceful and is aimed at producing energy.
There was no immediate Iranian government reaction to the letter or any discussion of it in Iranian media.
Cotton is a freshman senator who serves on the Senate Armed Services and Intelligence committees.