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A group of 13 current and incoming GOP House members are calling on Republican senators to oppose the omnibus spending bill released early Tuesday morning, threatening to “thwart” any policy priorities championed by Republican senators who vote for the legislative package.

The tactic also got an endorsement from House Minority Leader Kevin McCarthy (R-Calif.), who is aiming to become Speaker despite opposition from several of the letter’s signatories that threatens to keep him from the gavel. The GOP Leader pledged that bills from GOP senators who vote for the package would be “dead on arrival” if he is Speaker.

“Put aside the absurd spending and empowerment of Biden bureaucrats to continue their intentional abuse of power and dereliction of duty. It is the willingness of Senate Republicans to abandon for now the one leverage point we have – the power of the purse – to stop Biden’s purposeful refusal to secure and defend our borders that is most offensive,” said the letter sent Monday, led by Rep. Chip Roy (R-Texas).

The letter called the omnibus bill an “indefensible assault” on separation of powers, fiscal responsibility and civic decency. The bill, which was negotiated between Republicans and Democrats, needs support from at least 10 Senate Republicans in order to overcome a filibuster.

“Further, we are obliged to inform you that if any omnibus passes in the remaining days of this Congress, we will oppose and whip opposition to any legislative priority of those senators who vote for this bill – including the Republican leader,” the letter said. “We will oppose any rule, any consent request, suspension voice vote, or roll call vote of any such Senate bill, and will otherwise do everything in our power to thwart even the smallest legislative and policy efforts of those senators.”

The threat marks an escalation for the House GOP members who have long called for an omnibus spending bill to be delayed until after the GOP takes control of the House.

Rep. Scott Perry (Pa.), chair of the House Freedom Caucus, also signed the letter, along with Reps. Dan Bishop (N.C.), Andy Biggs (Ariz.), Matt Rosendale (Mont.), Matt Gaetz (Fla.), Ralph Norman (S.C.), Bob Good (Va.), Byron Donalds (Fla.) and Andrew Clyde (Ga.), and Reps.-elect Anna Paulina Luna (Fla), Andy Ogles (Tenn.), and Eli Crane (Ariz.).

The letter also warns of further fracturing of the Republican Party if the legislation passes with the help of Republican senators.

“Kill this terrible bill or there is no point in pretending we are a united party, and we must prepare for a new political reality,” the letter said.

Text of the omnibus spending package, which would last through the end of the 2023 fiscal year in September, was released early Tuesday morning. Both chambers of Congress are expected to vote on the legislation this week before government funding runs out on Dec. 23.

The sweeping 4,155-page bill includes $44.9 billion in funding for Ukraine and NATO allies, $2 million in “off-campus” security for lawmakers, Electoral Count Act reform and a measure to ban the Chinese-owned social media app TikTok from government phones.

Many of the signers of the letter have withheld support from McCarthy’s Speakership bid as they push him to adopt more aggressive tactics and rules changes that would empower rank-and-file members.  Biggs, Rosendale, Gaetz, Good, and Norman have all explicitly said or strongly indicated that they would vote against McCarthy in the Jan. 3 Speaker’s vote.

On Wednesday, McCarthy tweeted an endorsement of the tactics outlined in the letter.

“Agreed. Except no need to whip—when I’m Speaker, their bills will be dead on arrival in the House if this nearly $2T monstrosity is allowed to move forward over our objections and the will of the American people,” McCarthy said.

McCarthy had already echoed the concerns outlined in the letter and called for a continuing resolution into the new year so a Republican-controlled House could have leverage to push for spending cuts and other priorities. Some senators have chalked up his stance as a political move in order to earn support for the Speakership, though McCarthy has bushed off that assertion.

This story was updated at 11:28 a.m.