Defying the wishes of President Obama, six Republican senators Tuesday proposed an amendment to a defense spending bill that would introduce a new round of sanctions on Iran on the eve of diplomatic talks the president hopes will end Iran’s quest for a nuclear weapon.

The amendment, offered by Sen. Mark Kirk (R-Il.) and co-sponsored by five other Republican senators, including Senate Minority Leader, Mitch McConnell (R-Ky.) as well as Sen. Kelly Ayotte (R-NH), Sen. John Cornyn (R-TX), Sen. Lindsey Graham (R-SC) and Sen. Marco Rubio (R-FL), would target the remaining money Iran has in overseas bank accounts, most of which comes from the sale of oil. It would be attached to the National Defense Authorization Act, the annual bill that directs spending and policy for the U.S. military.

A draft of the amendment, obtained by The Daily Beast, would sanction any bank that allows Iran to spend revenue in overseas accounts on items besides food and medicine and lays out what steps Congress would need to see in any interim agreement with Iran.

The amendment also states any such interim agreement must require Iran to stop the enrichment of uranium, a condition of earlier U.N. Security Council resolutions. The administration has hoped to fudge this issue so far and has been discussing a temporary agreement that would allow Iran to do some enrichment while the larger deal is negotiated over a six-month period.

The introduction of the amendment comes on the eve of talks set to begin Wednesday in Geneva between Iran and China, France, Germany, Russia, the United Kingdom and the United States.

The move from Kirk and his fellow Republicans is a direct challenge to President Obama. For nearly a month, the president has asked Congress hold off on any new sanctions bill in order to allow the Geneva negotiations an opportunity to succeed.

On Tuesday, Obama met with Republican and Democratic senators to discuss the negotiations in Geneva. None of the lawmakers that met with Obama have signed on to the Kirk amendment on Iran. A White House statement issued after the meeting with those lawmakers said the president “has a responsibility to pursue the ongoing diplomatic negotiations before pursuing other alternatives.”

Kirk however said his amendment would give U.S. diplomats in Geneva the leverage they would need to strike a good deal. “Sanctions remain the best way to avoid war and prevent Iran from acquiring a nuclear weapons capability,” he said. “This proposal will give our diplomats the increased leverage they need to get a good deal at the negotiating table—a deal that peacefully brings Iran into full compliance with its international obligations.”

By setting out the conditions acceptable for an interim deal with Iran, the Kirk amendment gets to the heart of a dispute between Obama and American allies in the Middle East. In particular, Israel has warned against any sanctions relief on Iran until Iran begins to dismantle the thousands of centrifuges it has built in Natanz and Qom and begins to roll back its capability to quickly produce the highly enriched fuel needed for a nuclear weapon.