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Just after President Obama challenged Congress with his executive action on immigration, the administration may present congressional leaders with another tough nut 鈥� either a comprehensive Iran nuclear deal or an extension of nuclear talks with Tehran.

With foreign ministers from six world powers and Iran gathering in Vienna for the final hours of negotiations before a Monday deadline, signs are multiplying that something other than a breakdown of talks will occur. Either a broad framework of an agreement could be reached, or 鈥� perhaps more likely 鈥� some additional time will be announced to sort out remaining technical issues.

Secretary of State John Kerry, who had planned to leave Vienna for Paris on Friday, announced late in the day that he would remain overnight in Vienna to meet again with Iranian Foreign Minister Mohammad Javad Zarif and other senior officials involved in the talks.

One key reason the pressure is on is that negotiators know that opponents of a deal in Washington and Tehran are ready to strike in the event of no deal or of a nebulous extension.

The talks between Iran and the United States, Britain, France, Russia, China, and Germany were already extended once after an interim deal was reached a year ago.

鈥淏oth sides realize that a deal gets much harder after鈥� the Monday deadline, says Kelsey Davenport, director of nonproliferation policy for the Arms Control Association in Washington. 鈥淭here are domestic constituencies in both Washington and Tehran that would make reaching a deal after the 24th more difficult.鈥�

In Tehran, opponents of a deal are hard-liners who want to see no limits on Iran鈥檚 nuclear program. In Washington, the extremely wary constituency is Congress. Lawmakers have essentially been holding back in taking action, such as passing more sanctions on Iran.

On Thursday, GOP Sens. Marco Rubio of Florida and Mark Kirk of Illinois sent a letter to Mr. Obama citing their 鈥渁larm鈥� at 鈥渞eports that your administration plans to circumvent Congress鈥� and proceed 鈥渦nilaterally鈥� with an agreement. The senators say they fear the White House plans to grant Iran substantial sanctions relief in return for only 鈥渨eak and dangerous鈥� requirements for limiting Iran鈥檚 nuclear program.

The letter was cosigned by 41 other Republican senators.

But if a deal is struck, Obama is likely to hear howls of protest from more than just Republicans. Prominent Democrats, including New Jersey Sen. Robert Menendez, outgoing chairman of the Foreign Relations Committee, have also sent warning shots about a 鈥渨eak deal.鈥�

One problem some proliferation experts see is that congressional action that ends up tying the president鈥檚 hands or scuttling a deal (for example, by imposing a new round of sanctions) would leave the world blaming the US. Any follow-on consequences of a breakdown 鈥� from a breakup of international consensus about keeping Iran from attaining a nuclear weapon, to military action to deteriorate Iran鈥檚 nuclear facilities 鈥� would be laid at America鈥檚 feet.

If action initiated by Congress were to lead to 鈥渃oercive action鈥� against Iran, 鈥渢he question is, who does the world blame?鈥� says George Perkovich, an expert in nuclear nonproliferation and vice president for studies at the Carnegie Endowment for International Peace in Washington.

Responsibility for a breakdown would have 鈥渞eal consequences for maintaining international support鈥� on restraining Iran鈥檚 nuclear program, says Mr. Perkovich, who continues, 鈥淚 think the Iranians get that 鈥� but I don鈥檛 have a sense that many people on the Hill get that.鈥�

One option Congress would have in the event of a deal with Iran, or of an extension of talks, would be to pass new sanctions, but to make any backsliding by Iran the trigger for implementation. Sen. Bob Corker (R) of Tennessee, who will take over the chair of the Foreign Relations Committee, proposed something along those lines over the summer.

But others say most of what鈥檚 coming out of the Hill suggests Congress is intent on scuttling a deal that many members believe would be too lenient with Iran.

Congress is hearing the message of a 鈥渂ad deal in the making鈥� from a range of anti-Iran forces in Washington, from conservative pro-Israel groups to the Iranian opposition. Earlier this month, the National Council of Resistance of Iran took to Congress the finding of a secret investigation that Iran has undisclosed explosives research at its Parchin research facility.

Dylan Williams, director of government affairs for J Street, a lobbying group describing itself as 鈥減ro-Israel and pro-peace,鈥� says he has seen various 鈥渇lavors of legislation鈥� on Iran over recent months, ranging from 鈥渟traight-up new sanctions鈥� to initiatives on Iran鈥檚 human rights record or involvement in terrorism. But all of them, he says, seem to have a common objective.

鈥淭he intent in each of these cases,鈥� he says, 鈥渋s to frustrate this deal.鈥澛�

[Editor's note: The sub-headline of the original version of this story mischaracterized how opponents of a deal might react.]