Democratic Senator Chuck Schumer announced on Thursday he would vote against the Iran nuclear deal, in a blow to President Barack Obama's efforts to sell the measure.

The decision by the influential Jewish lawmaker was quickly matched by another leading Democrat, Eliot Engel, the ranking Democrat on the House foreign affairs committee.

"Advocates on both sides have strong cases fortheir point of view that cannot simply be dismissed," Schumer, the number three Democrat in the Senate, said in a statement

"This has made evaluating the agreement a difficult and deliberate endeavor," he admitted, following days of speculation about his intentions.

"I have decided I must oppose the agreement and will vote yes on a motion of disapproval."

Separately, Engel said that he simply did not trust Iran to uphold its end of the agreement to curtail its nuclear program in exchange for an easing of crippling economic sanctions.

The announcements were a serious blow to Obama, who is in the midst of an intense lobbying effort to get Congress to back his landmark agreement, arguing that to torpedo it would be to risk a wider Middle East war.