Republicans are flexing their muscles and threatening to block President Obama from cutting a nuclear deal with Iran on his own terms.
International negotiators have until Nov. 24 under an interim agreement to reach a deal with Tehran that would curb its nuclear program in exchange for sanctions relief, or seek another extension of talks.
A GOP Congress could doom what the president hopes will be a legacy foreign policy achievement.
Sen. Lindsey Graham (R-S.C.) took to the Senate floor on Thursday to ask for unanimous consent to schedule a vote on a bill that would give Congress final approval over any deal, or else reinstate tough sanctions on Iran.
Democratic Sen. Chris Murphy (Conn.) quickly rejected the request, arguing that scheduling a vote on the deal would be "premature at this point." He said it would "send a fairly chilling message" that U.S. officials at the table with Iran did not have full authority to negotiate an agreement.
But when Republicans take control of the Senate, they could move to pass that bill, or push legislation from Senate Foreign Relations Chairman Bob Menendez (D-N.J.) and Sen. Mark KirkMark KirkGOP senator: Don't link Planned Parenthood to ObamaCare repeal Republicans add three to Banking Committee Juan Williams: McConnell won big by blocking Obama MORE (R-Ill.) which would reinstate sanctions if Iran violates any deal.
Their bill also pledges military support for Israel if it decides to strike Iran's nuclear facilities, which it has threatened to do.
Senate Majority Leader Harry Reid (D-Nev.) delayed a vote on the bill earlier this year under pressure from the White House, which argued that it could sink any chance of reaching a deal. Sen. Mitch McConnell (R-Ky.), the next majority leader, though has expressed support for tough sanctions.
The president has already threatened to veto the legislation, but doing so would be politically risky. The bill already enjoys the support of 60 senators, including 16 Democrats, and there is sweeping support for a similar bill in the House.
A deal that's not supported by Congress or seen as weak could also hurt the 2016 Democratic presidential nominee, especially if it fails to prevent Iran from obtaining nuclear weapons capabilities.
Experts believe negotiators will extend their talks beyond the November deadline for several months, which would allow Republicans to pass Iran legislation before a deal is reached.
If negotiators do reach a deal before Republicans take power, the GOP can still try to stand in the way. Republicans could move a bill requiring congressional approval of any deal, pass legislation defunding implementation, or pass a non-binding joint resolution expressing disapproval.
Some experts said GOP pressure and oversight before a deal is reached would strengthen U.S. negotiators by creating clear red lines they could not cross in talks.
"In any negotiation, you need a good cop and a bad cop. People in the room need to have the backstop that they can point to," said Mark Dubowitz, executive director of the Foundation for Defense of Democracies.
Trita Parsi, founder and president of the National Iranian American Council, which supports the talks, disagreed.
"If the U.S. Congress is moving forward with sanctions legislation, it sends a signal that the U.S. is not serious in reaching a deal, and the Iranians are not going to entertain this process," he said.
"This is good cop and crazy cop.”
A major point of contention between lawmakers and the White House is whether Iran will be allowed to continue to enrich uranium. The interim deal, known as the Joint Plan of Action, relaxed sanctions in exchange for Iran limiting, but not ending, domestic enrichment of uranium.
Other issues that need to be negotiated include the duration of a final deal before Iran is treated like other nuclear powers, the nature and pace of sanctions relief, Iran’s ability to develop advanced centrifuges and improve its enrichment capability, and the fate of two nuclear facilities, the Arak heavy water reactor facility and the Fordo enrichment facility.
On Wednesday, Sens. Kirk and Menendez issued a statement seeking to set their expectations on those issues.
“We believe that a good deal will dismantle, not just stall, Iran’s illicit nuclear program and prevent Iran from ever becoming a threshold nuclear weapons state,” they said.
“If a potential deal does not achieve these goals, we will work with our colleagues in Congress to act decisively, as we have in the past,” they warned.
Robert Einhorn, senior fellow at the Brookings Institution and a former Obama adviser on Iran negotiations, warned that if sanctions legislation goes too far, it could risk unraveling international support for pressure on Iran.
"It's important to keep the pressure on Iran," said.
"But it has to be done in a skillful way to avoid fracturing the sanctions coalition that we need to sustain pressure on Iran."
This story was updated on Nov. 17 at 12:02 p.m.