WASHINGTON — Republicans in Washington are growing increasingly worried in the wake of a move by one of the GOP's biggest financial backers to break from President Donald Trump's agenda.

The decision by the conservative Koch network to be more selective in supporting candidates that back their free trade, small government policies, has spurred fear among GOP lawmakers that a major funding spigot will dry up, and they worry that the growing spat is dividing the party before the crucial midterm elections.

The network, which spends upward of $400 million each election cycle, with a large percentage spent advocating their issues and supporting Republicans for Congress, announced last weekend that the group is scaling back its political support and “raising the bar” for candidate endorsements.

That has rattled Republicans who have relied on Koch money to supplement their own campaign message and to be a reliable attack dog against Democratic opponents.

Sen. David Perdue, R-Ga., said he’s “very concerned” by the Koch’s new posture this election cycle and beyond.

“We need all the help we can get right now because the left is very clear because the presidential candidate they’re going to put up in 2020 is going to be very liberal,” Perdue said.

But the Koch organization has grown frustrated with the GOP, which is transforming into a more protectionist, nationalist party under Trump's leadership — a shift that Koch officials see as counter to their agenda.

The Koch network and the president have split on the issues of tariffs and immigration, and the group is trying to pressure lawmakers to stay committed to traditional Republican ideals.

“There are honest disagreements within the GOP on some topics, tariffs and trade obviously being one of them," said Chris Wilson, director of research, analytics and digital strategy for the 2016 presidential campaign of Sen. Ted Cruz, R-Texas. "And the Koch's strong free-market ideology doesn't fit well with the current set of policies being pushed by the White House."

The disagreement on trade, an issue central to both the president and the Koch network, has deepened a fissure in the party that already existed.

In a rare interview session with reporters Sunday, Charles Koch, the head of the organization, said he "regrets" having supported some Republicans in the past who "say they're going to be for these principles that we espoused and then they aren't." And he pledged to be more "selective" in supporting candidates going forward.

Trump took offense at the network’s message, tweeting that they are “highly overrated” and that he has “beaten them at every turn.”

The globalist Koch Brothers, who have become a total joke in real Republican circles, are against Strong Borders and Powerful Trade. I never sought their support because I don’t need their money or bad ideas. They love my Tax & Regulation Cuts, Judicial picks & more. I made..... — Donald J. Trump (@realDonaldTrump) July 31, 2018

....them richer. Their network is highly overrated, I have beaten them at every turn. They want to protect their companies outside the U.S. from being taxed, I’m for America First & the American Worker - a puppet for no one. Two nice guys with bad ideas. Make America Great Again! — Donald J. Trump (@realDonaldTrump) July 31, 2018

On Thursday, the president continued the attacks, tweeting:

Charles Koch of Koch Brothers, who claims to be giving away millions of dollars to politicians even though I know very few who have seen this (?), now makes the ridiculous statement that what President Trump is doing is unfair to “foreign workers.” He is correct, AMERICA FIRST! — Donald J. Trump (@realDonaldTrump) August 2, 2018

But as one Koch donor put it, each side is trying to protect its ideals.

“I think Trump is saying, whose agenda are you supporting? And I think it’s very much the same way for (the Kochs) in that they don’t want anybody defecting on any issue,” said a donor who spoke on the condition of anonymity.

The rift has begun to affect this year's midterm elections, and the way the Koch organization has approached the North Dakota Senate race, for example, has caused deep frustration among Republicans.

In May, a Koch-associated group placed a small-dollar mailer ad “thanking” the Democratic incumbent, Sen. Heidi Heitkamp, for voting to rollback banking regulations. And over the weekend, at their semiannual donor confab in Colorado Springs, the organization announced that it wouldn’t support Heitkamp's Republican challenger, Kevin Cramer, because he has been “inconsistent” on key issues of government spending and corporate welfare.

Republican senators are reluctant to speak critically publicly, understanding Trump’s high approval ratings among their base and worrying about exacerbating the divide.

“When you see ads for Heidi Heitkamp, that caused a little bit of a pause,” said one Republican senator who said senators have been asking a lot of questions of each other about what the Koch network’s plan is. “I think concerns have to do with fundraising more than anything else.”

The organization told reporters that, as of now, it is engaging in just four Senate races behind candidates who fit their mold of backing free trade and small government — in Florida, Missouri, Wisconsin and Tennessee. There was no mention of House races to reporters during the three-day seminar.

But two participants told NBC News that in closed-door sessions over the weekend, the organization laid out plans to support Republican candidates involved in 10 House races and two additional Senate races — West Virginia and Ohio — bringing the total of Senate races to six. A spokesman for the organization, James Davis, declined to confirm what was discussed behind closed doors.

But not on that list are some crucial states. North Dakota is considered to be atop the list of Republicans’ best opportunities to win a Democratic seat, followed by Indiana, which the Koch network has no plans of getting involved in at this stage.

Also absent from the list is Nevada, where incumbent GOP Sen. Dean Heller is facing a tough challenge from Democratic Rep. Jacky Rosen, and Arizona, where a three-way race for the primary is underway. But one participant said the Koch officials called the race “a mess” and have no intention of getting involved.

“Members of Congress do need to know that we’re raising the bar," Americans for Prosperity President Tim Phillips said in a phone interview. "What we would have accepted in the past — repeated votes to increase government spending, failure to lead at important moments such as when trade with the rest of the world is at stake — I think the rest of the country and the network is going to demand more.”

Some Republicans argue that the Koch’s more limited political engagement will have little impact.

“This disagreement is unlikely to hurt Republicans in the midterms as there are enough opportunities for the Koch network to back like-minded candidates while other groups and super PACS can put their money into the races where the Koch's don't,” Wilson said.

Rep. Mark Meadows, R-N.C., who has attended Koch seminar weekends in the past and has also become a staunch ally of the president’s, said that the Koch’s money is less relevant.

“In truth we’re not going to solve the American people’s problems with more money. We’re going to solve it by being unified and working on the people’s agenda between now and November,” he said in a phone interview with NBC News.

Not all lawmakers are concerned. Sen. Tim Scott, R-S.C., attended the most recent Koch weekend, and said afterward, "I’m probably an outlier, but I thought it was a fairly positive message.”

“It certainly is a tad bit of a paradigm shift that, rightfully so, is focusing on the issues more than anything else," he said. "They’re prioritizing around the issues that they think will make America better.”