House Minority Leader Nancy Pelosi said she believes Democrats will be able to uphold President Obama's veto if Republicans vote down his nuclear deal with Iran in September and she called the agreement "a diplomatic masterpiece."

Pelosi, D-Calif., said Democrats will rally behind the agreement even after the long August recess, when outside groups plan a media blitz aimed at sinking public support.

"More and more [Democrats] have conferred to me they will be there to sustain the veto," Pelosi told reporters on Thursday.

Congress has begun a 60-day review of the deal, which would lift sanctions against Iran in exchange for a reduction in its nuclear weapon capability. Republicans mostly oppose the agreement and with majorities in both the House and Senate, will likely garner enough votes to back a disapproval of the deal.

But two-thirds of lawmakers in both chambers would be needed to override a presidential veto.

The Obama administration, with the help of Pelosi, is working to convince Democratic lawmakers to vote to uphold the veto, which would keep the Iran nuclear deal alive.

Democrats have publicly shown a mixed reaction to the plan, with many questioning a 24-day window for Iran to agree to inspections and a provision allowing Russian to sell ballistic missiles to the Islamic Republic.

Pelosi said despite the concerns among some Democrats, most will back the deal.

"Our members have a firm foundation on why they support this," Pelosi said.

She called the accord, "a diplomatic masterpiece," but acknowledged it does not include everything the United States wanted.

"You have to weigh the equities," Pelosi said. "Every agreement is not a perfect agreement. But, it's a viable agreement. A pragmatic agreement."