For many Democrats, Sen. Chuck Schumer is the single most important vote in Congress on the Iran deal.

Outside of Sen. Bob Menendez, who was indicted on federal corruption charges in April, the New York senator has been the harshest Democratic critic of the Iran talks in the Senate. Earlier this year, he bucked the White House and joined with Senate Majority Leader Mitch McConnell (R-Ky.) to cosponsor a bill that threatened to impose new sanctions on Iran if no deal was reached. In January, he signed a letter to the White House warning that moderate Democrats would ignore the president’s concerns about passing new Iran sanctions legislation if no deal emerged after March 24.

In late March, he became an important ally to Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.) after agreeing to cosign the Iran Nuclear Agreement Review Act, a bill that paved the way for a congressional review of the deal despite initial opposition from the White House.

“Schumer was a huge get,” Jamil Jaffer, former chief counsel to the Senate Foreign Relations Committee, told Foreign Policy. “The real question is whether he will be able to stand up to massive White House pressure.”