Democratic Sens. Richard Blumenthal, Gary Peters and Ron Wyden announced Tuesday they would support the agreement. All three were seen as swing votes potentially leaning to a “no” vote on the international pact.

“Rejecting this agreement is fraught with unacceptable risk,” Blumenthal said in a statement. “There is no better deal available now. The present sanctions will soon become unenforceable, producing an economic windfall for Iran whether or not the United States accepts the agreement.”

Blumenthal’s support, alongside his colleagues, ensures that a Republican-backed congressional resolution of disapproval cannot pass the Senate now that the Obama administration has the support of 41 Democrats in the chamber.

Republicans, backed by deal critic Israel, had sought to sway as many Democratic senators to oppose the deal, but have succeeded in getting only four to join ranks, most recently Sen. Joe Manchin who announced his opposition earlier Tuesday.

“I don’t believe a vote against this deal forces us to abandon the diplomatic path. We must continue to pursue peace, but on terms that promise a lasting peace for the United States and our allies,” Manchin said in a statement.

The international deal struck by Iran and world powers provides Tehran with economic relief from biting sanctions in exchange for a decade of unprecedented inspections and curbs on its nuclear program.