The decision by one of the top Democrats in the U.S. Senate to oppose the nuclear deal between Iran and world powers was "disappointing but not surprising," White House spokesman Josh Earnest said on Friday.

President Barack Obama's hopes of preserving the nuclear deal were dealt a blow on Thursday when New York's Schumer came out against it even as the White House tried to draw attention to other lawmakers who are backing the agreement.

Earnest told reporters that the Obama administration worked closely with Schumer to explain details of the deal even before it was signed in an effort to gain his support.

"Ultimately, it didn't turn out that way. I don't think anybody was surprised," Earnest said.

Obama is struggling to gain congressional backing for the deal, which lawmakers must vote on by Sept. 17. Schumer's rejection means the Obama administration may have to ramp up its lobbying efforts on Capitol Hill.

Twelve members of the U.S. Congress, seven members in the House of Representatives and five in the Senate, have publicly come out in support of the agreement in recent days, showing Obama had made a persuasive case, Earnest said.

The administration is confident they can continue to build support before next month's deadline, he added.

(Reporting by Lindsay Dunsmuir, Doina Chiacu and Jeff Mason; Editing by Susan Heavey)