One remaining question mark is Sen. Chris Coons (D-Del.), who supports the deal but would still like to see a vote on the floor.

UPDATE, 6:06 p.m. -- Cantwell announced on Tuesday evening that she would support the deal, bringing the total number of votes to 42. With Cantwell's announcement, there are no undecided Senate Democrats left.

PREVIOUSLY --

WASHINGTON -- In the face of two months of fierce campaigning by opponents of the Iran nuclear deal, 41 Democrats have said publicly they will nevertheless support it. The tally gives President Barack Obama enough backing to stop a GOP effort to undo the deal in the Senate, without needing to exercise a veto, if all of those backers support a filibuster.

The American Israel Public Affairs Committee, the pro-Israel lobby group, spent millions on television ads and face-to-face pressure on Capitol Hill, yet lost by a far greater margin than had been expected. J Street, a progressive pro-Israel group, and the National Iranian American Council led the effort to rally lawmakers in support of the nuclear deal, but with comparatively minuscule budgets -- outside of the Democratic Party apparatus, there was nowhere near the same financial firepower behind the organized campaign to support the deal.

“What we have to rely on are grassroots networks and me being able to get on the horn with a bunch of rabbis and hopefully they’re paying attention," Obama lamented in July. "You work with what you got.”

The horn, it turned out, was quite effective. On Tuesday morning, Democrats were three votes short of the needed 41, with four Senate Democrats outstanding. Before noon, three had declared their support, with the fourth, Sen. Maria Cantwell (D-Wash.), still undeclared.

One remaining question mark is Sen. Chris Coons (D-Del.), who supports the deal but added to CNN: "I think it would be really regrettable if we didn't ultimately go to the floor and cast our votes for or against this deal." If that means he would oppose the filibuster, Democrats would need Cantwell to come along.

When the Senate reconvenes Tuesday afternoon, Republican leadership is expected to schedule a vote on a resolution of disapproval of the nuclear agreement negotiated in July between Iran, the U.S. and five world powers. Republicans are expected to vote as a bloc against the accord, leaving it to the Democrats to save the diplomatic agreement.

The initial goal for Obama and Democrats in support of the nuclear deal was to get 34 votes -- just enough to prevent opponents from overriding a presidential veto. With that benchmark realized last week, the minority party rallied for the additional seven votes to block the passage of the resolution of disapproval.

Although the effort to kill the nuclear agreement appears set to fail, the fight in Washington over how to handle Iran’s nuclear program and broader activity in the Middle East is far from over.

Sens. Mark Kirk (R-Ill.) and Bob Menendez (D-N.J.) have said they intend to bring forth additional sanctions legislation targeting Iran’s support for terrorist groups in the region. And Sens. Ben Cardin (D-Md.) and Michael Bennett (D-Colo.) announced last week that they will introduce a bill that will expedite the process for sanctioning Iran if it commits acts of terror against the U.S., require the administration to report to Congress on how Iran is spending the money it receives from sanctions relief, and provide Israel with increased military aid to offset the boon to Iran’s economy.

Four Democratic senators have broken with their party by opposing the deal:

Ben Cardin (Md.) @SenatorCardin

After subjecting his colleagues to two months of attack ads by agreeing to a 60-day waiting period for a vote, Cardin, the top Democrat on the Senate Foreign Relations Committee, said Friday he will oppose the deal. Cardin, in the end, was unable to withstand the pressure. Over his career, he has not been much of a risk taker: Cardin went straight from law school to the Maryland House of Delegates, where he served for 20 years until it was his turn to go to Congress, where he served another 20 years until it was his turn to move to the Senate. He became the top-ranking member of the Foreign Relations Committee when the more senior Sen. Bob Menendez (D-N.J.) was indicted for corruption. His ultimate capitulation was unsurprising to observers.

Bob Menendez (N.J.) @SenatorMenendez
A known Iran hawk and the author of nearly every piece of recent sanctions legislation against Iran, Menendez announced Aug. 18 that he would not support the Iran deal. He also slammed the agreement in his initial statement: “I’m concerned the redlines we drew have turned into green-lights; that Iran will be required only to limit rather than eliminate its nuclear program, while the international community will be required to lift the sanctions, and that it doesn’t provide for anytime-any-place inspections of suspected sites. The bottom line is: The deal doesn’t end Iran’s nuclear program -- it preserves it.”

Chuck Schumer (N.Y.) @SenSchumer
The incoming Senate Democratic leader was the first member of his party in the Senate to officially come out against the deal. “I will vote to disapprove the agreement, not because I believe war is a viable or desirable option, nor to challenge the path of diplomacy. It is because I believe Iran will not change, and under this agreement it will be able to achieve its dual goals of eliminating sanctions while ultimately retaining its nuclear and non-nuclear power,” Schumer wrote in a statement.

While this position did not come as a surprise, it was met with harsh rebuke by progressive Democratic groups, who have vowed to withhold funding from Schumer and other Democrats who oppose the nuclear agreement.

Joe Manchin (W.Va.) @Sen_JoeManchin

The red-state Democrat earlier said he was leaning toward backing the agreement, but wouldn't support a filibuster. “That would stop all of you from knowing how we voted on this,” said Manchin, speaking at a town hall meeting at the University of Charleston. “My Democratic caucus does not like that position at all. I just think it’s too important to circumnavigate that thing because of political posturing, and I won’t do that.”

Overall, he had spoken positively about the deal. "Everybody says there is a better deal," he told MSNBC’s “Morning Joe” in late July. "What options are on the table or even basically discussed I could consider voting for?"

"I'm leaning very strongly to saying, OK, let's try going along with the P5+1,” he continued, referring to the U.S., Britain, France, Germany, Russia and China, which collectively negotiated the deal with Iran.

Manchin backed two rounds of sanctions legislation in the past two years that were opposed by the Obama administration, which claimed that the timing of the sanctions could upend the negotiations with Iran. However, the senator eventually withdrew his support for one of the bills, citing a desire to give the negotiators a chance to succeed. On Tuesday morning, he decided to announce his opposition.

The senators who have publicly pledged to back the Iran nuclear agreement are: Tammy Baldwin (D-Wis.), Michael Bennet (D-Colo.), Richard Blumenthal (D-Conn.), Cory Booker (D-N.J.), Barbara Boxer (D-Calif.), Sherrod Brown (D-Ohio), Maria Cantwell (D-Wash.), Tom Carper (D-Del.), Bob Casey (D-Pa.), Chris Coons (D-Del.), Joe Donnelly (D-Ind.), Dick Durbin (D-Ill.), Dianne Feinstein (D-Calif.), Al Franken (D-Minn.), Kirsten Gillibrand (D-N.Y.), Martin Heinrich (D-N.M.), Heidi Heitkamp (D-N.D.), Mazie Hirono (D-Hawaii), Tim Kaine (D-Va.), Angus King (I-Maine), Patrick Leahy (D-Vt.), Amy Klobuchar (D-Minn.), Edward Markey (D-Mass.), Claire McCaskill (D-Mo.), Jeff Merkley (D-Ore.), Barbara Mikulski (D-Md.), Chris Murphy (D-Conn.), Patty Murray (D-Wash.), Bill Nelson (D-Fla.), Gary Peters (D-Mich.), Jack Reed (D-R.I.), Harry Reid (D-Nev.), Bernie Sanders (I-Vt.), Brian Schatz (D-Hawaii), Jeanne Shaheen (D-N.H.), Debbie Stabenow (D-Mich.), Jon Tester (D-Mont.) Tom Udall (D-N.M.), Mark Warner (D-Va.), Elizabeth Warren (D-Mass.), Sheldon Whitehouse (D-R.I.), and Ron Wyden (D-Ore.).

This is a developing story and will be updated if more senators announce their support.

Clarification: This article has been revised to reflect that efforts to support the deal did not lack organization in proportion to the deal's opponents, but rather did not match the money spent by opponents.

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