Sen. Ben Cardin’s announcement that he will vote against the Iran deal throws a potential speedbump into President Obama’s growing victory parade, just as Democrats seemed to be building the numbers to keep a resolution of disapproval from getting through Congress.

“This is a close call,” the Maryland Democrat wrote in a Washington Post op-ed Friday, “but after a lengthy review, I will vote to disapprove the deal.”

Cardin, who serves as ranking member on the Senate Foreign Relations Committee, is only the third Democratic senator to break with Obama over the agreement where Iran curbs its nuclear ambitions in exchange for an easing of economic sanctions, joining Sens. Charles E. Schumer (D-N.Y.) and Bob Menendez (D-N.J.). So far, 37 Democrats and Democrat-leaning Independents have pledged to back the deal.

But what may be more important than the numbers is the timing of Cardin’s announcement.

Earlier this week, Obama secured enough support in the Senate to guarantee he would be able to sustain a veto of a resolution of disapproval on the Iran deal, if it ever reached his desk.

Since then, the count of pro-deal Democrats has been inching up. On Thursday, Sens. Cory Booker (D-N.J.), Heidi Heitkamp (D-N.D.) and Mark Warner (D-Va.) — each of whom came under intense lobbying pressure to oppose the deal — stepped forward to pledge their support, and political insiders started to wonder whether Obama might be within reach of an even bigger diplomatic coup: gathering enough backers in the Senate, 41, to kill the disapproval resolution in the Senate though a filibuster, obviating the need for a veto showdown.

WATCH: Secretary of State John Kerry has harsh words for the Iran nuclear deal oppososition (AP)

Up until Friday morning, Cardin was the most influential senator remaining in the undecided set. His breaking with the deal, coupled with the knowledge that Obama already has the backing to successfully veto a resolution of disapproval, may now give others still on the fence political cover to oppose the deal.

To date, administration officials and pro-deal lobbyists have been very cautious when speculating about whether Senate Democrats might be able to filibuster the resolution when Congress returns from recess next week. It’s too far from a sure thing — and even if it were, they don’t to want to jinx anything.

Even though the ultimate fate of the Iran deal is set, the jockeying for votes continues unabated.

The deal’s opponents want to collect as many detractors as possible, to strengthen their argument that the White House can’t really claim “victory” if only a veto-sustaining plurality, not a majority, of both chambers of Congress backs the Iran pact. Senate Democrats are also coming under fire from Republicans over the prospect that they may not allow for an up-or-down vote, which would guarantee the resolution of disapproval would be sent to Obama’s desk for a veto because there are enough votes among GOP senators.

To avoid all that, Obama’s supporters need four of the six remaining undeclared Senate Democrats to come out in favor of the Iran nuclear deal.

The undeclared are a variegated set. There’s Sen. Richard Blumenthal (D-Conn.), who says the deal is far from perfect — but also agrees with Obama, that no deal could be. He is coming under intense pressure from anti-deal groups, especially because he is the last undecided Jewish Senate Democrat. There’s also Sen. Joe Manchin (D-W.Va.), who has praised the administration’s diplomatic efforts, but has stayed mum on how he will vote. Both have left themselves enough room to be potential support votes on Iran.

Then there’s Sen. Michael Bennet (D-Colo.), who is up for reelection next year, and Sens. Maria Cantwell (D-Wash.) and Gary Peters (D-Mich.), who have been keeping such a low profile that it’s difficult to know what they’re thinking. Peters was an original co-sponsor of the Kirk-Menendez sanctions bill on Iran, which was ultimately set aside. And finally there’s Sen. Ron Wyden (D-Ore.), who has spoken about how skeptical he is that the deal will hold.

Cardin came under heavy pressure from lobbyists, constituents, friends and the administration to pick a side.

Administration officials have charged that opposing the deal is tantamount to favoring war with Iran because critics have not presented a viable alternative to bring Tehran back to the table and keep Iran under surveillance and free of nuclear weapons.

Cardin too, didn’t lay out a surefire plan to secure a better deal — though he pledged in his op-ed to bring up legislation “designed to strengthen the Iran Nuclear Sanctions Review Act” and said he favored a diplomatic solution.

“What happens if Congress rejects the JCPOA? No one can predict with certainty the consequences,” Cardin wrote. “Our European partners understand that they cannot effectively act without the United States. Iran understands that if it accelerates its nuclear program it will ignite international action against it. And Iran needs U.S. sanctions relief. Ultimately, it is in everyone’s interest to reach a diplomatic solution.”