As Congress heads into a summer recess set to feature intense lobbying by both sides, many Democrats have already endorsed the agreement. On Thursday, after Obama gave a fiery speech warning of harsh consequences should Congress vote it down, two more Democrats who were wavering came out in support: Sens. Kirsten Gillibrand of New York and Jeanne Shaheen of New Hampshire.
Republicans have scheduled a mid-September vote on a resolution to reject the Iran deal, which is expected to pass, and have eyed the August recess as the time to build up public pressure against the agreement. But with the latest string of Democrats coming out in favor of the deal, it is growing increasingly difficult for Republicans in the House and Senate to secure the votes they need -- 44 and 13 respectively -- from Democrats to help them override the veto Obama's promised.
Even the announcement Thursday night by New York Sen. Chuck Schumer, slated to become the Democratic leader in the next Senate, might not be enough to slow the White House's momentum.
He's the most influential Democrat to come out in opposition to the deal, a stance long sought by the agreement's opponents, many of whom are his constituents.
But the decision to make his position public on the heels of a string of endorsements was being seen in the White House as a signal that it has a veto-proof number of supporters and it was safe for Schumer to oppose the deal without jeopardizing the President's agenda.
"We're very confident that we can hold that veto, with the Democratic Caucus in the House and also the Senate," Ben Rhodes, the President's deputy national security advisor, told CNN on Wednesday.
In the House, the list of Democrats publicly opposing the deal is relatively short so far. It does, however, include a handful in senior positions -- New York's Steve Israel, Nita Lowey and, as of Thursday night, Eliot Engel among them, though House Minority Leader Nancy Pelosi has endorsed the deal.
The resistance from members of the President's party comes chiefly from those representing districts with significant Jewish constituencies, many of whom are fiercely opposed to the administration's foreign policy in the Middle East.
Republicans, however, are hopeful that Schumer's voice of opposition combined with that of some key House Democrats from Schumer's home state, could serve as a major influence on the roughly dozen or so undecided Senate Democrats.
In addition, New Jersey Democrat Bob Menendez, the former chairman of the Senate Foreign Relations Committee, has made it clear he has a host of problems with the deal and is expected to vote with Republicans.
Maryland's Ben Cardin, current Senate Foreign Relations Committee ranking member and currently undecided, is another important vote.
There are "a couple of -- maybe three -- key senators on the Democratic side that, depending on which way they go, will sway a number of others," said Tennessee Republican Bob Corker, current chairman of the Senate Foreign Committee, though he declined to name them.
For now, the volume of support House and Senate Democrats are giving the White House some momentum heading into the summer break, though Schumer's announcement could through a wrench into the works.
During the recess, outside groups opposed to the deal will be pouring tens of millions of dollars into campaign-style efforts to sway wavering Democrats and shape public opinion to ramp up pressure around the fall debate in Congress. They will be broadcasting ads, sending out mailings and visiting members' home-state offices.
Republican campaign eclipsed
Republicans, for their part, had planned to focus their August message on the dangers of approving the deal with Iran, seeing town hall meetings with constituents as an opportunity to gin up opposition as was done in similar settings against Obamacare.
But a series of videos showing Planned Parenthood officials discussing the sale of fetal tissue has triggered a new controversy over federal funding for the non-profit group. The conservative base is ramping up a campaign of its own to push GOP leaders not to approve any government funding bills that include federal money for Planned Parenthood.
That debate is taking some attention away from Republicans' effort to turn up the heat on Democrats, who are themselves using the August break to argue that the GOP fight on the spending bill could lead to another government shutdown.
To counter the anti-deal effort in whatever shape it takes, the White House is keeping in close touch with undecided members and pledging they will have an open line to the President even as he heads off for a two-week vacation on Martha's Vineyard.
One concern for the administration with several weeks to go is the many Democrats who have told CNN they could hold out right until the vote is called. For Democrats who are already deeply skeptical that the Iranian government is committed to implementing the agreement, any signal from leaders in Tehran that undermines the deal during that time could jeopardize efforts to win their support. And Schumer's opposition now coming weeks before the vote could provide cover for any who worried about breaking with their party.
Several of those asked about the deal in recent days have indicated they still aren't ready to speak out.
"I've got work to do," Democratic Sen. Claire McCaskill of Missouri told reporters after Wednesday's classified briefing, saying she had a series of calls lined up with top diplomats to press them to resolve some outstanding concerns.
"I'm doing my due diligence," Sen. Mark Warner of Virginia, another Democrat wrestling with how to vote, told CNN.
Pennsylvania Sen. Bob Casey said he has attended briefings led by administration officials but is also setting up his own sessions with outside experts to give him additional analysis. "I'm about a third of the way through the process."
Risk in Obama's hardball speech
Obama took one last parting shot before leaving Washington, hitting critics of the deal hard in a speech Wednesday. He put the decision facing members in this vote on a par with the vote to go to war with Iraq. He insisted blocking the deal would result in a march to war.
Though much of his attack was directed at Republicans -- he accused them of making "common cause" with hardliners in Tehran chanting "Death to America" -- it's unclear whether the President's sharpened message will backfire with those Democrats that he most needs to convince.
Though at least five House Democrats and three senators have come out in support of Obama since his speech, others are taking a more equivocal stance.
"I voted against the Iraq war. I do not see a comparison between this vote and the Iraq vote," Cardin, who is still evaluating the deal, said on Thursday.
Corker predicted that some of the Democrats still making up their minds would be alienated by the Obama's words.
"It probably infuriated some of the people that are on the bubble on the Democratic side of the aisle that are really trying to make up their mind," he said.