WASHINGTONObama administration officials hoping to end the nuclear standoff with Iran not only face a nation legendary for hard-line negotiating, they also must deal with members of Congress who may be just as unyielding.

In talks with Iran set to resume in Geneva in mid-October, the White House must weigh two competing challenges: coaxing Tehran to stop uranium enrichment and other nuclear work, and winning support from a Congress that is skeptical of easing sanctions against Iran.

In an era when Congress is divided on almost everything, the desire to bash Iran is nearly universal on Capitol Hill, uniting tea party conservatives such as Sen. Ted Cruz (R-Texas) and liberals like House Minority Leader Rep. Nancy Pelosi (D-San Francisco).

Since only Congress can permanently lift the bruising sanctions it has imposed on Iran, lawmakers can torpedo any deal if they believe the White House is giving too much to Iran's pragmatic new president, Hassan Rouhani, or his hard-line boss, supreme leader Ayatollah Ali Khamenei.

"We have a tremendous amount of leverage," said Rep. Ed Royce (R-Fullerton), chairman of the House Foreign Affairs Committee and co-author of the toughest Iran sanctions legislation ever adopted by the House.

If the Senate passes the bill, which the House approved in July by a vote of 400 to 20, the U.S. would seek to cut Iran's oil exports — which account for 80% of Iranian government revenue — to near zero by punishing purchasers. The bill would also take a long step toward clamping a total trade embargo on Iran.

Royce made it clear that he doesn't trust the conciliatory overtures Rouhani made during his recent visit to the United Nations, or during his 15-minute phone call with President Obama on Sept. 27.

"Their best hope is to draw things out through negotiations, while they continue their nuclear work," Royce said.

Obama and Rouhani are both eager to avoid war, and have incentive to compromise. Diplomats say a possible deal might allow Iran to enrich uranium to low levels for peaceful purposes under strict oversight by the International Atomic Energy Agency, or import nuclear fuel from Russia or other countries.

To some extent, the aggressive approach on Capitol Hill sets up a good-cop, bad-cop dynamic between the White House and Congress that can provide potential leverage for U.S. officials if the talks in Geneva lead to further negotiations, experts said.

U.S. negotiators can tell the Iranians that if they don't give ground on their nuclear program, "those crazy people on the Hill might do anything," said Mark Dubowitz, a sanctions specialist at the Foundation for Defense of Democracies, a pro-sanctions advocacy group.

But Congress could go too far by imposing even harsher sanctions, enraging Iranian hard-liners and jeopardizing the new diplomatic opening. Some influential lawmakers, for example, are urging Congress to press ahead with more sanctions if Iran doesn't offer immediate concessions on several fronts, not just nuclear development.

"So long as Iran continues to pursue a nuclear weapons capability, build longer-range ballistic missiles, sponsor terrorism around the world and abuse human rights, the Senate should impose maximum economic pressure on Iran to give diplomacy a chance to succeed," Sen. Mark Steven Kirk (R-Ill.) said in a statement.

The administration has sought to appeal to both sides. On Thursday, Wendy Sherman, the State Department's third-ranking official, told the Senate Foreign Relations Committee that sanctions "should not disappear any time soon." But she added that the administration might offer Iran other short-term relief if it scaled back its nuclear work.

Sherman said officials were looking for ways Iran could "build confidence" and provide time for diplomacy to work. "We know that deception is part of the DNA" in Iran, she said.

Without congressional backing, Obama's diplomatic options are limited.

He can halt sanctions imposed by executive order, and he can temporarily suspend sanctions imposed by law, citing the needs of U.S. national security. But if he continues to suspend sanctions in ways that Congress doesn't support, he risks a blistering political attack.

Obama would most likely need to convince Congress that Iran has complied fully with tough U.S. legal requirements before lawmakers would permanently lift sanctions.

Congress also has leverage over European Union sanctions that have proved enormously effective. Tehran is eager to rejoin the Brussels-based international financial transaction system known as SWIFT so it can again move money around the globe.

But even if Iran is permitted back into the system, financial companies might be wary of cooperating with Tehran until the U.S. gives its blessing. That's because so-called secondary sanctions imposed by Congress bar foreign companies that do business with Iran from transacting commerce with any U.S. firm, a huge penalty for many international companies.

The long reach of U.S. sanctions law means the White House "really has to treat Congress as a full partner on this issue," said Dubowitz, who has been an advisor to lawmakers on the issue.

Some analysts say the dynamics of American politics make it easy for Congress to add sanctions and tough to remove them. Lawmakers who vote to ease sanctions on a longtime adversary might come under fire as weak on national defense. They're less likely to be blamed if Congress undermines White House peace negotiations.

A vote to continue or strengthen sanctions "is pretty cost-free for Congress," said George Perkovich, a nuclear specialist at the nonpartisan Carnegie Endowment for International Peace.

Since 2010, bipartisan majorities in Congress have voted to impose increasingly strict sanctions despite White House warnings that being too heavy-handed would limit U.S. diplomatic options and undermine cooperation from key European and Asian governments.

But the combination of U.S., U.N. and European Union sanctions have halved Iran's oil exports, cut the country's financial sector off from international banking and helped cause rampant inflation. Rouhani has made it clear that his chief goal is to get the sanctions lifted as soon as possible.

U.S. officials believe Iran is enriching uranium and developing components with an eye toward someday building a nuclear bomb. Iran insists it is only developing peaceful nuclear power.

Administration officials say they will require real concessions from Iran before easing the sanctions that have forced Iran to negotiate. Many U.S. lawmakers, in contrast, sound much like Israeli Prime Minister Benjamin Netanyahu, who has warned the administration against Iran's mollifying talk.

After visiting Obama at the White House this week, Netanyahu suggested that sanctions should be tightened unless Iran agreed to permanently halt uranium enrichment, ship its enriched uranium out of the country, dismantle its underground Fordow enrichment facility, and close the Arak reactor facility, which outsiders fear is designed to develop plutonium-bomb capability.

paul.richter@latimes.com