WASHINGTON — (CNN) President Barack Obama is on the verge of a major foreign policy victory.

Supporters of the Iran nuclear agreement are now just one vote shy of locking in enough support to preserve the deal after two Senate Democrats said Tuesday they would back it. Just about an hour apart, Sens. Chris Coons of Delaware and Bob Casey of Pennsylvania announced their support.

Amidst growing momentum for the deal, the top Democrat on the Senate Foreign Relations Committee, Ben Cardin of Maryland, predicted that by the end of the week, Obama will have secured the 34 Senate votes he would need to sustain his veto of a resolution of disapproval Republicans are planning to bring up later this month.

The steady and growing Democratic support comes despite nearly unified opposition from congressional Republicans and a heavy and expensive lobbying campaign by outside interest groups who fear the agreement isn’t strong enough and will allow Iran to eventually build a nuclear bomb.

“I am voting to support this agreement not because I think it is perfect, or because I believe it is the mechanism to end nuclear proliferation in the region. I am voting for this agreement because it is our most credible opportunity now to lead a global community in containing an existential threat while preserving America’s ability to use economic power and military might to successfully dismantle a nuclear program should diplomacy fail,” Coons, a member of the Senate Foreign Relations Committee, said in an address at the University of Delaware.

“I will vote to support the implementation of the Joint Comprehensive Plan of Action because I believe it is the best option available to prevent Iran from acquiring a nuclear weapon,” Casey said in a 17-page statement explaining his position. “It places strict limitations on Iran’s nuclear program, requires robust monitoring and verification measures, and grants relief only from nuclear sanctions in exchange for verified actions on Iran’s part.”

Cardin, speaking to a group of Johns Hopkins University students even before Coons and Casey made their public announcements, predicted ultimate success.

“The President will have at least 32 committed Democrats on his side by the end of today and that number looks like will clearly get to the 34 number by the end of the week,” he said.

Cardin, who is influential on international affairs, has not said how he will vote. He worked with Republicans to ensure Congress would get a vote on the deal, something the Obama administration initially opposed and has warned will kill the deal. He lamented that the lobbying “has been intense on both sides.”

“I think it’s a tough call,” said Cardin. “I think there are high risks either way.”

Cardin is a senior Jewish senator, and some of the most vocal opposition to the Iran deal has come from Jewish constituents and pro-Israel organizations.

More senior House Democrats also endorsed the Iran agreement on Tuesday. Rep. Adam Smith of Washington, the top Democrat on the House Armed Services Committee, released a statement saying he backed the agreement. Maryland Rep. Elijah Cummings, the highest-ranking Democrat on the House Oversight Committee, also said he would vote for it. Illinois Rep. Bobby Rush and New York Rep. Hakeem Jeffries announced their support, too.

House Democratic leadership aides continue to express confidence they will secure enough votes to uphold a veto in the event that a resolution blocking the deal is sent to the President’s desk.

Only two Democratic senators, Chuck Schumer of New York and Robert Menendez of New Jersey, have announced opposition to the deal. Just under a dozen Democratic senators remain unannounced.

Coons said he spoke to all the remaining undecided senators, but declined to predict how they will vote.

Only one Republican senator is considered a possible vote in favor of the bill. Sen. Susan Collins of Maine will announce her decision next week, according to her staff.

“She remains concerned about some key aspects of the agreement, including the lack of a good inspection regime, which will make verification extremely difficult, and the lifting of the embargoes on conventional arms and intercontinental ballistic missile technology,” her office said in a recent statement.

Supporters of the deal hope to get at least 41 votes to block the disapproval resolution from even coming up for a final vote in the Senate.

“I think that will be a very close call,” Coons said when asked about that Tuesday.

House Republicans could schedule a vote as early as next week on the GOP resolution to scuttle the deal, but no decisions have been made yet, according to a senior House GOP leadership aide. If it becomes clear that 41 or more Democrats signal they will filibuster the resolution in the Senate, the House is likely to move ahead with a vote soon after Congress returns from its summer recess.

Senate leaders have not said when they will vote. There is a statutory deadline of Sept. 17, but votes may be scheduled several days before that because a Jewish holiday and a Republican presidential debate are complicating the timetable.

The-CNN-Wire ™ & © 2015 Cable News Network, Inc., a Time Warner Company. All rights reserved. (Photo: CNN)