On Iran Talks, Congress Could Play "Bad Cop" WASHINGTON - When Iranian diplomats sit down in Geneva next Tuesday with the United States and five other world powers for a new round of talks about Iran's nuclear program, Congress will not have a seat at the table. But that does not mean it will not have a voice. With a tough, new Iran sanctions bill teed up in the Senate, following the overwhelming passage of similar legislation by the House in July, lawmakers are poised to do one of two things: They could tighten the screws on Iran's leaders in a way that helps produce a nuclear deal. Or they could foul up delicate diplomacy at a crucial moment. The Senate banking committee, under pressure from Secretary of State John Kerry, agreed to put a brief pause on its bill to avoid spoiling the first bargaining session in Geneva. But the committee's chairman, Senator Tim Johnson, Democrat of South Dakota, has told the Obama administration he plans to move forward with the bill in coming weeks. That sets up the prospect of Congress voting for draconian new sanctions against Iran just as the West is forming a judgment about whether Iran's new president, Hassan Rouhani, is serious about reaching an agreement that would ease concerns about Iran's nuclear ambitions enough for the United States to lift existing sanctions. "We know the sanctions are starting to take a toll on the regime," said Senator Mark S. Kirk, an Illinois Republican and Iran hawk, who has sponsored bills to cut off Iran's access to international financing. This is the moment to ratchet up the pressure, not dial it back. It is not the first time that Congress has played the heavy in the diplomatic dance between the United States and Iran. On several occasions in recent years, it has passed legislation - sometimes over the objections of the White House - that has forced President Obama to be tougher than he might otherwise have been on the Iranian government. This time, though, Capitol Hill's influence looks to be more important, and less predictable. Although Mr. Rouhani was elected with a mandate to negotiate relief from sanctions, there is a deep latent hostility to diplomacy among hard-liners in Iran. Some Iran watchers worry that if Congress were to pass new sanctions prematurely, it could provoke a conservative backlash in Tehran that would doom the new leader's efforts. "These negotiations are going to be tremendously complex," said Cliff Kupchan, an Iran expert at the Eurasia Group, a risk consulting firm. The ultimate train wreck would result from Congress moving forward on this sanctions bill before diplomacy has a chance to produce results. To its defenders, Congress is the unsung hero of the pressure campaign on Tehran. Time and again, Mr. Kirk said, the White House has tried to block or water down sanctions - those that blacklisted Iran's central bank, for example - only to take credit for them later, when the legislation prodded the European Union to take similar action. It is a role that Congress shares with Prime Minister Benjamin Netanyahu of Israel, who warned the United Nations that Mr. Rouhani was a "wolf in sheep's clothing," and urged the United States to ignore his conciliatory words and redouble the pressure on Iran. "Netanyahu's speech was widely ridiculed in this town," Mr. Kupchan said, "but it largely reflects the views of many members of Congress." Some administration officials are forthright in acknowledging the benefit of Congress being the "bad cop." Even as she requested a delay in the Senate bill, Wendy R. Sherman, the under secretary of state who is conducting the negotiations, told the Senate Foreign Relations Committee that she planned to invoke the specter of Congress with the Iranians. "I want to be able to say to Iran," Ms. Sherman said, "this is your opportunity. Come on the 15th of October with concrete, substantive actions that you will take; commitments you will make in a verifiable way; monitoring and verification that you will sign up to; to create some faith that there is reality to this, and our Congress will listen. The problem, say former administration officials, is that this round of talks is unlikely to produce a tangible proposal. While Iran may signal a commitment to negotiate, they say, it is not expected to offer to suspend its enrichment of uranium or mothball suspect facilities. "If people on the Hill are waiting for dramatic results on the evening of Oct. 16 to decide whether to pass sanctions, that's wrong," said Robert Einhorn, a former special adviser for nonproliferation in the State Department. One shouldn't set up a situation where unless major progress is being made, we impose new sanctions. Even if Congress were to act now, the sanctions would not take effect for 180 days, raising questions about their impact, said Mark Dubowitz, executive director of the Foundation for Defense of Democracies. With Congress paralyzed by the impasse over the budget and fiscal policy, however, squeezing Tehran is one of the few things on which Democrats and Republicans can agree. The House bill, which aims to drive Iran's diminished oil exports down to zero, passed by a 400-to-20 vote. The Senate version would cruise to victory, too, though the government shutdown could bog down its march to a vote. In 2011, the Senate passed a bill aimed at Iran's oil exports by a unanimous vote. Those sanctions crippled Iran's economy and helped bring Mr. Rouhani to power. "The notion that we can hold off pressure from the Hill, in the absence of anything concrete from the Iranians, is an illusion," said Dennis B. Ross, a former senior Obama adviser on Iran.