Liberal Democrats have mounted a furious offensive to convince Senate Democrats to oppose legislation the White House warns could kill a nuclear deal with Iran.

In moves that appeared coordinated, House Minority Leader Nancy Pelosi Nancy PelosiSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Trump is betting big on the suburbs, but his strategy is failing 'bigly' Trump orders flags at half-staff to honor 'trailblazer' Ginsburg MORE (D-Calif.) announced her opposition to a bill that would give Congress a vote on the emerging deal. Minutes later, Sen. Barbara Boxer Barbara Levy BoxerThe Hill's Morning Report - Presented by Facebook - Biden, Harris launch Trump offensive in first joint appearance Bottom line Polls show big bounce to Biden ahead of Super Tuesday MORE (D-Calif.) urged the Senate Foreign Relations Committee to postpone a planned vote next week.

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“Diplomacy has taken us to a framework agreement founded on vigilance and enforcement, and these negotiations must be allowed to proceed unencumbered,” Pelosi said in a statement. “Senator Corker’s legislation undermines these international negotiations and represents an unnecessary hurdle to achieving a strong, final agreement.”

Earlier on Wednesday, a coalition of liberal groups including MoveOn.org Political Action and Democracy for America threatened to retaliate against Democrats who support the review legislation championed by Senate Foreign Relations Committee Chairman Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.).

The moves appeared to shift calculations on the politics of supporting Corker’s bill.

The measure appeared close to gaining veto-proof support on Monday night after Sen. Charles Schumer Chuck SchumerVideo of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Graham signals support for confirming a Supreme Court nominee this year Pelosi orders Capitol flags at half-staff to honor Ginsburg MORE (D-N.Y.) reiterated his support for the bill.

But now several of the bill’s Democratic co-sponsors won’t say whether they’ll vote to approve it on Tuesday at a panel markup. They are demanding that Republicans tone down provisions opposed by the White House.

“The big question is what will Corker do,” one Democratic aide said. “If Democrats agree on amendments and Corker won’t go with them, it could be problematic in terms of getting broader Democratic support in committee.”

Delaware Sen. Chris Coons, one of the bill’s eight Democratic co-sponsors and a member of the Foreign Relations Committee, wants to remove language that would require the administration to certify that Iran no longer supports terrorism.

Sen. Chris Murphy (D-Conn.), another panel member, wants to change the legislation so that President Obama could roll back sanctions without having to wait for Congress to review and approve the deal.

Democratic aides have also floated a possible amendment that would shorten the mandated 60-day congressional review period.

Florida Sen. Bill Nelson has said he wants unspecified changes to the bill.

“He’s intending to make some modifications that will be acceptable to the White House,” said an aide to the senator.

Adam Bozzi, a spokesman for Sen. Michael Bennet (Colo.), another Democratic co-sponsor, said Wednesday his boss is “open to changes.”

Neither Nelson nor Bennet sit on Foreign Relations, but they could seek changes on the floor or ask Maryland Sen. Ben Cardin, the panel’s ranking Democrat, to push for amendments in committee.

President Obama and senior White House officials have reached out to senators in recent days to sell the emerging deal with Iran and dissuade them from voting for any legislation that could undercut negotiations.

Obama called Corker on Tuesday but a White House spokeswoman declined to provide any details from their conversation.

Josh Earnest, the White House press secretary, suggested to reporters on Wednesday that the president hopes that senators can craft compromise language.

Obama told New York Times columnist Tom Friedman Saturday he hopes “we can find something that allows Congress to express itself but does not encroach on traditional presidential prerogatives.”

But Republicans share little optimism that Obama wants to hatch an agreement that will allow Corker-Menendez to pass with strong bipartisan support, despite the president’s chat with Corker.

The administration threatened to veto the legislation in February, casting a cloud over the prospect of meaningful negotiations.

“The White House is still fundamentally opposed to the underlying structure of the bill,” said a Senate aide

A Senate aide said some Democrats are beginning to second-guess their support for the bill in the wake of the administration’s announcement last week that it had reached a preliminary deal.

“Now that it’s less theoretical and less abstract, now that we have a preliminary agreement, we may be seeing a collision course with reality. Members feel a little differently about the bill and want to offer amendments,” said the Democratic aide.

Heavy pressure from grassroots advocacy groups has raised the stakes of bucking Obama on what could be the biggest legacy foreign policy accomplishment.

“When pressure is applied, people are going to react,” the aide said of new calls from Democrats to amend Corker’s bill.

Another Democratic aide disputed that Democrats are looking for an excuse to vote against the measure. The aide argued Democrats are instead looking to grow support within their caucus.

Jordain Carney and Kristina Wong contributed to this report.