The signing of the interim agreement, congressional aides said, could cut both ways. While some senators might be more inclined to give diplomacy a chance, others might be troubled by the terms of the six-month deal. The full text of the agreement has not yet been released, arousing the suspicions of critics, though the White House said on Monday that it would soon be made available to lawmakers.

Mr. Obama and other senior officials have met repeatedly with lawmakers to defend their diplomatic efforts and to try to stop the rush to sanctions. They cite an intelligence assessment that sanctions could undermine the negotiations. And they argue that Congress can always act swiftly to impose sanctions if the talks do collapse.

“My preference is for peace and diplomacy, and this is one of the reasons why I’ve sent the message to Congress that now is not the time for us to impose new sanctions,” Mr. Obama said to reporters on Monday after meeting with Spain’s prime minister, Mariano Rajoy. “Now is the time for us to allow the diplomats and technical experts to do their work.”

Much will depend on the Senate majority leader, Harry Reid of Nevada, who has so far resisted pressure to allow a vote on a sanctions bill. Mr. Reid is balancing a record of robust support for Israel with an equally strong alliance with the White House. Aides say Mr. Reid will not bring a bill to the floor before the State of the Union address on Jan. 28.

Democrats, they say, recognize the delicacy of Mr. Obama’s signing a veto on the Iran bill, especially if Congress delivers the first veto override of his presidency on a matter that is so clearly a presidential prerogative. But Democrats said the current lull can hold only for a matter of weeks, not months.

Prominent Democrats like Senator Michael Bennet of Colorado, the chairman of the Democratic Senatorial Campaign Committee, have recently thrown their support behind the bill. Aides say enough Senate Democrats would support the sanctions bill to override a presidential veto, and the House probably has a veto-proof margin as well.