Reid's remarks come after Sen. Patty Murray (D-Wash.) pulled her bill from being voted on.
But the administration says it doesn't have the agreements between Iran and the IAEA.
The House bill comes in the wake of a San Francisco shooting that launched a political firestorm.
But the legislation still faces challenges from both sides.
The bill would roll back certain parts of the EPA's regulations on coal ash waste disposal.
The letter comes after Susan Rice acknowledged the so-called “side” agreements.
Sen. Joe Manchin is the only Democrat on the letter.
House GOP leaders have won over Rep. Steve King (R-Iowa).
The move comes as Republican senators seek an investigation into Planned Parenthood.
Senate Republicans lined up the floor Wednesday to voice their concerns.