In an effort to reassert control over the domestic political debate surrounding sensitive negotiations on Iran’s nuclear program, the White House penned a letter Saturday night warning senators to hold back on legislation that would detract from the president’s ability to affect and approve a final agreement with Iran.

The letter, written by White House Chief of Staff Denis McDonough to Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.), reiterates a veto threat of the bill, while insisting that Congress will have a say in reviewing and affecting the ultimate outcome. But in far more detailed and foreboding terms than normal, McDonough lays out the administration’s concerns should Corker’s Iran Nuclear Agreement Review Act of 2015 end up becoming law.

The bill, he writes, would “likely have a profoundly negative impact on the ongoing negotiations — emboldening Iranian hard-liners, inviting a counter-productive response from the Iranian majiles; differentiating the U.S. position from our allies in negotiations; and once again calling into question our ability to negotiate this deal.”

“Put simply,” adds McDonough, “it would potentially make it impossible to secure international cooperation for additional sanctions, while putting at risk the existing multilateral sanctions regime.”