The letter, drafted by Senator Tom Cotton, a freshman from Arkansas, and signed by all but seven members of the Senate Republican majority, warned Iran that a deal with Mr. Obama might not stick. “The next president could revoke such an executive agreement with the stroke of a pen, and future Congresses could modify the terms of the agreement at any time,” said the letter, whose existence was reported earlier by Bloomberg News. Mr. Cotton said he drafted the letter because Iran’s leaders might not understand America’s constitutional system. He also said the terms of the emerging deal were dangerous because they would not be permanent and would leave Iran with nuclear infrastructure. He noted that four Republican senators who may run for president signed his letter and added that he tried without success to get Democrats to sign. “The only thing unprecedented is an American president negotiating a nuclear deal with the world’s leading state sponsor of terrorism without submitting it to Congress,” he said on CNN. The letter revived an old debate about what role Congress should have in diplomacy. Jim Wright, the Democratic House speaker during Ronald Reagan’s presidency, was accused of interfering when he met with opposing leaders in Nicaragua’s contra war. Three House Democrats went to Iraq in 2002 before President George W. Bush’s invasion to try to head off war. And Nancy Pelosi, the House Democratic leader, went to Syria in 2007 to meet with President Bashar al-Assad against the wishes of the Bush administration, which was trying to isolate him. An agreement with Iran would not require immediate congressional action because Mr. Obama has the power to lift sanctions he imposed under his executive authority and to suspend others imposed by Congress. But permanently lifting those imposed by Congress, as Iran has sought, would eventually require a vote. Rather than wait, Republicans, joined by several Democrats, drafted legislation aimed at forcing Mr. Obama to submit the agreement to Congress. But when Senator Mitch McConnell of Kentucky, the Republican majority leader, moved to advance that legislation for a vote, Democrats who support it balked at taking action before the talks with Iran concluded. Mr. McConnell backed off, but the bill may be revived if a deal is reached. Among the Republicans who declined to sign Mr. Cotton’s letter was Senator Bob Corker of Tennessee, the Foreign Relations Committee chairman, who has been working with Democrats on Iran legislation. “We’ve got a bipartisan effort that’s underway that has a chance of being successful, and while I understand all kinds of people want to weigh in,” he said, he concluded that it would not “be helpful in that effort for me to be involved in it.” Some Democrats, like Representative Brad Sherman of California, said the letter and other moves risked making it a party-line issue, in which case it would be impossible to muster a two-thirds vote to override a presidential veto. “The number of Democrats not willing to follow the president’s lead is reduced when it becomes a personal or political issue,” he said.