According to Reuters, Democratic Senators Richard Blumenthal, Gary Peters, Ron Wyden and Maria Cantwell announced they would support the agreement, just as lawmakers returned to Washington from a month-long summer recess.

Forty-two votes is one more than the minimum needed in the 100-member Senate to block a Republican-backed resolution of disapproval of the nuclear deal, announced on July 14.
That would spare Obama the embarrassment of having to use his veto power to protect a deal reached with five other world powers, seen as a potential legacy foreign policy achievement for his administration.

Obama had been guaranteed enough votes to sustain a veto once he reached 34 ‘yes’ votes in the Senate, but backers say avoiding the veto process would send an important message to Iran, and the world: Washington is unified behind it.

White House spokesman Josh Earnest said the administration was 'gratified' by the growing support for the nuclear accord.

All of the senators supporting the deal are Democrats or independents who caucus with them. Every supporter in the House of Representatives is a Democrat.

Senator Joe Manchin on Tuesday became the fourth Senate Democrat voting against the deal. At least 17 House Democrats have also said they would vote with Republicans against it.

To block the resolution, deal supporters would need at least 41 senators to vote in favor of using the Senate's filibuster procedural rule to keep a disapproval resolution from advancing; IRNA reported.