As it attempts to hold and possibly expand upon its newly won congressional majority, the House Democrats' campaign arm is planting an early marker in hopes of preventing primary challenges against sitting incumbents.

The Democratic Congressional Campaign Committee (DCCC) – in a letter sent Friday to more than 100 political firms – clearly stated that it won’t work with, or recommend to House Democratic candidates, any vendors who help to oust incumbents.

MODERATE DEMS FUME OVER OCASIO-CORTEZ INCUMBENT HIT LIST

“The core mission of the DCCC Is electing House Democrats, which includes supporting and protecting incumbents. To that end, the DCCC will not conduct business with, nor recommend to any of its targeted campaigns, any consultant that works with an opponent of a sitting Member of the House Democratic Caucus,” the committee spelled out in a memo obtained by Fox News.

It’s common practice for both the Democratic and Republican House and Senate re-election arms to protect incumbents running for re-election. But the DCCC’s memo, sent extremely early in the election cycle, is a clear signal to Democratic political firms that the millions in contracts dished out each election by the party committee’s independent-expenditure arm will be off limits to them if they work with insurgent candidates. And that could put a big dent in a primary challenger’s ability to take on an incumbent lawmaker.

The move could help moderate Democrats running for re-election in crucial swing districts, and even some controversial liberal lawmakers.

Among those behind the memo were moderate Rep. Henry Cuellar of Texas and progressive firebrand Rep. Ilhan Omar of Minnesota, according to National Journal. Both lawmakers could face 2020 primary challenges.

In 2018 primaries, then-Rep. Michael Capuano of Massachusetts lost his bid for an 11th term in Congress to now-Rep. Ayanna Pressley. And socially conservative Democratic Rep. Dan Lipinski nearly lost in his primary to progressive challenger Marie Newman. Both Newman and Pressley were helped in their bids by well-known political shops.

Rep. Alexandria Ocasio-Cortez of New York, who’s become a nationally known progressive leader, defeated then-House Democratic Caucus Chairman Joseph Crowley running a low-budget campaign. She could face a primary challenge of her own in 2020. But she’s teaming up with a liberal group that helped her topple Crowley to now suggest a new round of primary challenges in 2020 against establishment House Democrats.

There’s a history of party committees warning political shops to stay away from primary challengers. Most recently, the House and Senate Republican re-election arms in 2014 cut off Jamestown Associates after the firm targeted GOP incumbents.