WASHINGTON � After months of delicate negotiations with the Iranian regime, President Obama this week will face a high-stakes confrontation with defiant lawmakers � including members of his own party � who are intent on influencing diplomacy over the future of Iran�s�nuclear program.


With supporters closing in on a veto-proof majority, the Senate Foreign Relations Committee will formally draft legislation Tuesday that would give Congress some authority over lifting sanctions against Iran, a precondition for Iran�s partial dismantlement of its nuclear complex.


Senate Republicans are trying to shut down efforts by Democrats to weaken the legislation and by Republicans to strengthen or complicate it, in hopes of preserving bipartisan support for a measure that the president fears would disrupt continuing diplomacy between Iran and six world powers, including the United States.


Even as Mr. Obama chastised lawmakers for complicating the negotiations, his administration began a scramble to win them over. To brief senators and their staffs, it dispatched the secretary of state, John Kerry; the energy secretary, Ernest J. Moniz; and other members of Mr. Obama�s national security team.


The administration, at the very least, wants Congress to hold off on any legislative action until the end of June, the deadline for turning this month�s interim framework into a detailed accord.


Members of the committee talked throughout the weekend in search of a midpoint between congressional prerogative and executive branch desires for them to stand down, without fundamentally altering the bill.


�I want to work with them so that Congress can look at this deal when it�s done,� Mr. Obama told reporters Saturday after the Summit of the Americas in Panama. �What I�m concerned about is making sure that we don�t prejudge it, or those who are opposed to any deal whatsoever try to use a procedural argument essentially to screw up the possibility of a deal.�


The president also castigated Senate Republicans, first for their �open letter� to the leadership of Iran warning that Congress might not abide by any deal, and then for suggestions that the Ayatollah Ali Khamenei�s interpretation of the interim nuclear accord reached early this month � emphasizing that inspectors would not have unfettered access to Iran�s military sites and that sanctions would be lifted immediately after the final deal was signed � was more credible than Mr. Kerry�s.


At a convention center just yards from Panama Bay, Mr. Obama alluded to the Cold War adage that �politics ends at the water�s edge� even as he took a sharp poke at Republicans. �Partisanship has crossed all boundaries,� he said, adding, �It needs to stop.�


For now, the White House � which is increasingly realizing that it cannot prevent some form of congressional oversight � must pin its hope less on Democratic efforts to dilute the legislation than on Republican efforts to toughen it so much that it loses its veto-proof support. At least 50 amendments by both parties are ready to go.


�The more we see the true agenda of the right wing coming out, which is war with Iran, I think the better it is for us to try and stop this, because I know Americans don�t want another war,� said Senator Barbara Boxer of California, a senior Democrat on the Foreign Relations Committee.


But the bill�s supporters � Democrat and Republican � say its effect on both the negotiations and the application of a final agreement are being overstated. �My message has been, �Spend time selling the deal instead of trying to stop congressional oversight,��� said Senator Jeff Flake of Arizona, one of a handful of Republicans who have not denounced outright an impending deal with Iran.


The bill in question, originally introduced in February by Senator Bob Corker of Tennessee, the Foreign Relations chairman, and Senator Bob Menendez, Democrat of New Jersey, enjoyed bipartisan support and seemed on pace to attract even more because its central concern was congressional prerogative. But the politics of the bill were complicated almost immediately, when Prime Minister Benjamin Netanyahu of Israel gave a speech to Congress against the wishes of the White House.


Next, Senator Tom Cotton, Republican of Arkansas, wrote the open letter to the Iranian government last month. Then, in another twist, Mr. Menendez was indicted and stepped down from the committee, leaving Senator Benjamin L. Cardin of Maryland, who is far less hawkish, in charge of the Democrats� role in the bill.


Supporters of the bill say that none of its nine Democratic co-sponsors have withdrawn and that there are plenty of other potential supporters to get to the 67 votes needed to override a veto.


Senator Richard Blumenthal of Connecticut, one of the nine, said the White House had yet to make its case that the bill would impede talks, but he suggested that he might consider a delay as negotiators try to complete the deal with Iran.


�I�m coming back firmly committed to a congressional role in this process,� he said, �but feeling strongly about talking to colleagues about improvements and when in the next three months we really need to consider this.�


The Iran review legislation is complex and often misunderstood. It would prevent the president from waiving any economic sanctions against Iran for 60 days as lawmakers review a final accord. After that review, lawmakers could vote to approve or disapprove the lifting of sanctions Congress imposed in 2010 � or take no action. The president would then be able to veto that resolution of disapproval.


Supporters of the bill say the measure would not stop Mr. Obama and the five other nations negotiating with Iran from concluding an accord limiting Iran�s�nuclear program�in exchange for a loosening of sanctions. It would do nothing to stop the European Union or United Nations from lifting its sanctions, or the president from waiving sanctions imposed by executive action. In a phased loosening of economic penalties, a final lifting of congressional sanctions could be put off until after Mr. Obama left office.


An override of a veto on the Iran Nuclear Agreement Review Act would not mean the votes would necessarily be there to override a subsequent veto of a resolution against the final lifting of sanctions, supporters say.


But White House officials say any action could have major repercussions as diplomats try to turn the ambitious framework reached this month into a final agreement by the end of June.


Officials are still holding out hope for persuasion. Mr. Kerry plans to conduct two closed-door briefings this week: one on Monday for House members, and a Tuesday briefing for senators.


�What we�re looking for is not to have Congress interfere with our ability, inappropriately, by stepping on the prerogatives of the executive department of the president,� Mr. Kerry said Sunday on the NBC News program �Meet the Press.�


Mr. Kerry has repeatedly asserted that all of the elements in the American text outlining the �parameters� of an eventual accord were agreed to by the Iranian side in the closed-door negotiations in Lausanne, Switzerland. On Sunday, Mr. Kerry said Russia, one of the United States� negotiating partners, agreed that the American text reflected the current state of the negotiations.


But also on Sunday, Ali Akbar Salehi, the head of Iran�s Atomic Energy Organization, restated Iranian expectations that economic sanctions would be removed quickly if a final accord was reached rather than suspended in phases as American officials have insisted.


Democrats on the committee, in close consultation with the White House, have focused on two issues. They want to relax the prohibition on Mr. Obama to waive economic sanctions, and they want to remove language tying approval of the accord to certification that Iran will no longer provide support for terrorist attacks on Americans.


But the biggest threat to the bill might come from committee Republicans, whose amendments could strengthen Mr. Obama�s argument that the bill would torpedo international negotiations. Senator Marco Rubio, Republican of Florida, is expected announce his candidacy for president on Monday and then fly back to Washington on Tuesday to introduce an amendment making approval of the deal dependent on Iran�s recognition of Israel�s right to exist.


Senator Johnny Isakson of Georgia, another committee Republican, will push an amendment demanding Iranian compensation for the victims of the hostage taking at the United States Embassy in Tehran 35 years ago.


�As we debate our foreign policy toward Iran, it seems more appropriate than ever that we compensate the victims of the Iran hostage crisis, who were forced to endure unimaginable fear, despair and torture for 444 days,� he said.


By The New York Times