FIRST ON FOX: A group of House conservatives are putting Senate Majority Leader John Thune, R-S.D., on notice: quickly pass a Trump-backed election bill or expect the House of Representatives to block every Senate measure.

Two dozen House Republicans, led by Rep. Randy Fine, R-Fla., are vowing to oppose any Senate bill until the House-passed Safeguarding American Voter Eligibility (SAVE) America Act clears the upper chamber.

"We made a promise to the American people. It’s time to deliver," the Republicans wrote in an open letter to Thune, first obtained by Fox News Digital. "Consider this our filibuster."

It’s a notable division between Republicans controlling the majorities in both houses of Congress.

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House Republicans have threatened for weeks to derail Senate legislation until President Donald Trump signs the measure into law. The SAVE America Act would require proof of citizenship to register to vote and photo identification for voters at the ballot box.

It comes as the Senate kicked off a marathon process Tuesday to debate the SAVE America Act, though the measure is ultimately expected to fail given unanimous opposition from Senate Democrats and Sens. Lisa Murkowski, R-Alaska, and Thom Tillis, R-N.C.

The group of House conservatives is calling on Senate leadership to pursue a talking filibuster to steer around the 60-vote requirement and pass the bill with a simple majority. However, internal divisions among Republicans have kept the conference from pursuing that approach. Thune has also warned a talking filibuster could backfire on Republicans if Democrats were to saddle the bill with Democrat-authored amendments.

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Despite clear math problems in the Senate, the group of House conservatives said they will continue to ratchet up the pressure. They also slammed Thune for beginning debate on the bill without having the votes to advance the measure to a vote on final passage.

"Continuing the same old kabuki shows is unacceptable, and the American people deserve better," Fine told Fox News Digital in a statement. "Majority Leader John Thune can say whatever he wants about my colleagues and me holding the line, but we won’t tolerate Washington’s games any longer."

"In the last election, the American people overwhelmingly elected President Trump and gave him and the Republican Party a mandate to Make America Great Again. Core to that mandate was a promise to restore confidence in the security of our elections — to guarantee that only Americans vote in them," the House lawmakers wrote in the letter.

It’s not clear the defecting group has the numbers to immediately derail Senate legislation with significant bipartisan support. 

Forty-one conservatives revolted Tuesday on the House floor against Senate legislation that would reauthorize a program supporting small businesses. The measure still sailed through the House after nearly all Democratic lawmakers backed it.

However, House conservatives’ threats could still hamper efforts by Speaker Mike Johnson, R-La., to steer party-line legislation through the chamber.

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With just a razor-thin majority, Johnson can currently only afford to lose one House Republican on any legislation that does not have Democrats’ support.

"On Sunday, President Trump announced that he will not sign any additional legislation until the SAVE America Act is passed out of the Senate. The President has also called on the Senate to use the talking filibuster to secure passage of the SAVE America Act immediately, superseding everything else. We agree," the letter said. "Until that occurs, we, the undersigned, are prepared to vote NO on any Senate bill on the House Floor."

A Senate GOP aide shot back at the House Republicans over their effort, saying, "Republicans fighting Republicans over congressional procedure is definitely a recipe for midterm success." The aide added that Thune's office has not received the letter as of the time of publication of this story.

It comes weeks after a group of House Republicans pushed Johnson on a lawmaker-only call to reject any Senate-led legislation until the SAVE America Act was passed.

"If we're going to go to war against our own party in the Senate, there may be implications to that," Johnson said at one point, according to people on the call. "So we want to be thoughtful and careful."

The letter notably did not make a carve-out for a Department of Homeland Security (DHS) funding bill, despite the White House assuring that Trump would make an exception for a spending bill to end the partial government shutdown.