The weekend deal spelling out how Iran will roll back its nuclear program in exchange for sanctions relief buttresses the Obama administration’s argument Tehran may be prepared for a grand diplomatic compromise averting the potential for war. But it has done little to sway skeptical lawmakers determined to levy new sanctions against Iran.

With world powers and Iran set to start the clock on their six-month interim agreement Jan. 20, a parallel showdown looms between President Barack Obama and Congress over legislative action each side says has serious implications for the chances of diplomatic success. Obama warns adding more sanctions could kill negotiations; legislators insist they’re the only way to ensure Iran keeps its word. Much could depend on Tehran quickly making good on its commitments.

“Now is not the time to impose new sanctions,” Obama said Monday as he met with Spanish Prime Minister Mariano Rajoy.

White House press secretary Jay Carney said the fact Iran’s government was now implementing what has been agreed “demonstrates that at the very least, testing whether or not Iran is serious is the right thing to do.”

The question of sanctions is essentially a tactical dispute over the best way to achieve a shared goal: preventing Iran from acquiring nuclear weapons and delivering an existential threat to Israel, while fundamentally reshaping the power dynamics of the Middle East. But for the Obama administration, the pressure from Congress has proven a constant headache at precisely the moment Iran’s moderate-sounding President Hassan Rouhani is offering unprecedented flexibility in talks.

The administration reached a milestone in its strategy Sunday. The U.S. and its five negotiating partners — Britain, China, France, Germany and Russia — secured a deal with Iran articulating how the Islamic republic will scale back its uranium enrichment program, halt progress at a plutonium plant and open up key sites to daily inspectors beginning next week. In exchange, world powers outlined how they will phase in $7 billion worth of relief from international sanctions that have crippled Iran’s economy. The full agreement has yet to be made public.

The agreement “will advance our goal of preventing Iran from obtaining a nuclear weapon,” Obama declared in a statement Sunday. He reiterated a threat to veto any new sanctions legislation from Congress, saying such action risks “derailing our efforts.” Carney added Monday that new sanctions are “wholly unnecessary” because Congress could always act later.

The implementation agreement provides the nuts and bolts to November’s breakthrough interim deal with Iran, an arrangement that can be extended by six additional months. Negotiators hope to replace it with a comprehensive accord this year ending the threat of a nuclear-armed Iran.

Iranian reformers and moderate conservatives welcomed the agreement Monday, while hardliners repeated their opposition. Iran’s leaders say uranium and plutonium activity is designed solely for peaceful nuclear energy and medical research purposes, but the United States and many other countries fear Tehran is covertly advancing toward atomic weapons capability. Israel and Iran’s Sunni Arab rivals such as Saudi Arabia have been most vocal in their skepticism of diplomacy, their concerns echoed by a growing chorus in Congress.

Fifty-nine senators now back the latest proposed U.S. sanctions package, which they say would increase the pressure on Iran to make concessions and fully dismantle — not simply slow down — the entire nuclear program. The count takes sanctions proponents closer to being able to push a bill through Congress and override even a presidential veto. The House overwhelmingly supports additional economic pressure on Tehran.

No sanctions vote is expected imminently. Congressional aides said top proponents such as Democratic Sens. Bob Menendez of New Jersey and Chuck Schumer of New York were waiting for the best opportunity to push the issue, with much of the focus now on building a stronger coalition. Some Senate aides cited early February as the earliest possibility.

The legislation under consideration would blacklist several Iranian industrial sectors and threaten banks and companies around the world with being banned from the U.S. market if they help Iran export any more oil. The provisions would take effect if Tehran violates the interim deal or lets it expire without a final agreement.

Several sanctions advocates said nothing this weekend changed their minds.

“If Iran is committed to comprehensively addressing its nuclear program, there is no reason such legislation shouldn’t be welcomed,” said Rep. Ed Royce, R-Calif., the House Foreign Affairs Committee chairman.

Sen. Mark Kirk, R-Ill., who co-drafted the bill, said new sanctions were needed to “ensure this process leads to the peaceful dismantlement of Iran’s nuclear program.”

Another declared supporter, Sen. Bob Casey, D-Penn., cited Iran’s past record of negotiating in bad faith and its U.S.-declared status as the world’s leading state sponsor of terrorism, but said he’d consult with American intelligence and the administration on how to advance national security interests.

Significant opposition in the Senate remains. Influential Democratic such as Dianne Feinstein of California and Carl Levin of Michigan have backed the administration’s call for a pause to new sanctions while international inspectors gauge Iran’s adherence to the deal. Senate Majority Leader Harry Reid held off a vote during defense bill negotiations before Christmas and hasn’t signaled when the issue might reach the floor.

Congressional aides said they expected little-to-no movement among legislators as a result of this weekend’s implementation agreement. The administration briefed Congress on some of the details Monday, participants in the conference call said. Obama will meet Senate Democrats to discuss priorities for 2014 on Wednesday night, officials said, and Iran is likely to come up.

With the Senate likely to hold off on action for at least the next couple of weeks, Iran will get some time to show even its most hardened detractors that its attitude has changed after years of ignoring U.N. Security Council demands over its nuclear program, export of weapons to countries such as Syria and ballistic missile development.

If Tehran proves its compliance to international inspectors, it would strengthen the case for patience and perhaps even ease some of the distrust between the United States and Iran dating back to the 1979 Islamic Revolution and subsequent U.S. embassy hostage crisis. If Iran cheats or denies monitors the promised access, it will provide ammunition to critics who’ve said all along that Tehran cannot be trusted. Such an event would strengthen calls for more sanctions.

Beyond the Obama administration’s opposition, other governments are also wary about seeing years of diplomacy undermined by a rush to new U.S. sanctions. The interim agreement forbids new nuclear-related sanctions by the United States, but is ambiguous about what the suspended measures currently being debated would mean. Iran’s foreign minister has said the sanctions would kill negotiations, and its parliamentarians have threatened to respond in kind.

(AP)