The Republican-on-Republican blood feud developing in West Virginia is only over a single House seat, but the outcome of Tuesday’s primary between Representatives David McKinley and Alex Mooney will signal the direction of a potential Republican majority in Congress: Will it be a party of governance or one purely of ideology, driven by former President Donald J. Trump?

Redistricting and West Virginia’s shrinking population forced the state’s Republican Legislature to pit Mr. McKinley, a six-term Republican with a pragmatic bent, against Mr. Mooney, who has served four terms marked more by conservative rhetoric than legislative achievements.

Mr. McKinley has the backing of much of the state’s power structure, including its governor, Jim Justice, and, in recent days, its Democratic senator, Joe Manchin III. Mr. Mooney, however, may have the endorsement that matters most: Mr. Trump’s — in a state that gave the former president 69 percent of the vote in 2020.

Neither candidate could exactly be called a moderate Republican, but Mr. McKinley thought his primary bid would be framed around his technocratic accomplishments, his support for the bipartisan infrastructure bill that was co-written by Mr. Manchin and his attentiveness to a state used to — and still in need of — federal attention.

On Thursday, he and Governor Justice were in the state’s northern panhandle, not for a campaign rally but to visit a high-tech metal alloy plant.

Mr. Mooney’s campaign does not go for nuance. His is built around one thing: Mr. Trump’s endorsement.

The former president sided with Mr. Mooney after Mr. McKinley voted for the infrastructure bill as well as for legislation to create a bipartisan commission to examine the Jan. 6, 2021, attack on the Capitol — legislation that was filibustered by Republicans in the Senate.

“Alex is the only candidate in this race that has my complete and total endorsement,” Mr. Trump says in a radio advertisement blanketing the state. The former president goes on to blast Mr. McKinley as a “RINO” — “Republican in name only” — “who supported the fake infrastructure bill that wasted hundreds of billions of dollars on the Green New Deal” and Speaker Nancy Pelosi’s “phony narrative” on Jan. 6 that went “against the interests of West Virginia.”

A television advertisement also featuring Mr. Trump tells viewers that Mr. Mooney defended the former president from Ms. Pelosi’s “Jan. 6 witch hunt.”

Sensing that any high-minded campaign on accomplishments was simply not going to work, Mr. McKinley has hit back at “Maryland Mooney” as a carpetbagger — he once headed the Maryland Republican Party and ran for office in New Hampshire — who is under investigation by the House Ethics Committee for charges that he improperly used campaign dollars and staff for personal gain.

Most remarkably, Mr. McKinley has turned to a Democrat, Mr. Manchin, for his closing argument.

“Alex Mooney has proven he’s all about Alex Mooney, but West Virginians know that David McKinley is all about us,” Mr. Manchin says in a McKinley campaign ad. He also calls Mr. Mooney a liar for suggesting that Mr. McKinley supported the far-reaching climate change and social welfare bill that Mr. Manchin killed.

All of this is somewhat extraordinary in a state where federal largess has made politicians like the now-deceased Senator Robert C. Byrd and his protégé, Mr. Manchin, folk heroes. But the state has changed in the Trump era, and loyalties have hardened, said Scott Widmeyer, co-founder of the Stubblefield Institute for Civil Political Communications at Shepherd University in Shepherdstown, W.V.

“We’ve seen heated political races, but I don’t think anything has been as nasty and down and dirty as this one,” he said. “Republicans are eating their own.”

Institute officials invited both candidates to a debate, but only Mr. McKinley accepted. They then suggested that the candidates come separately to town hall meetings. Only Mr. McKinley accepted.

Mr. Mooney is the one evincing confidence, however. Mr. McKinley entered the race at a structural advantage. The state’s newly drawn district includes 19 of the 20 counties Mr. McKinley previously represented and only eight of the 17 counties in Mr. Mooney’s current district. Mr. Mooney’s biggest population center, the capital in Charleston, was sent to Representative Carol Miller, the only other West Virginian in the House.

But Mr. Trump is popular in every West Virginia county, and on the power of his name, Mr. Mooney has been posting polls from national and local outfits showing him up by double digits ahead of Tuesday’s primary.

Aides close to Mr. McKinley say the race will be close, and as long as Mr. Trump does not swoop into the state at the last minute for a get-out-the-vote rally, either candidate could still win a low-turnout affair. One campaign official said many voters who long ago abandoned the Democrats but not their Democratic Party registration have been re-registering as independents or Republicans to vote against Mr. Mooney.

Jonathan Kott, a former spokesman and adviser to Mr. Manchin, said the Democratic senator has “a genuine friendship and working relationship” with Mr. McKinley, a point Mr. Manchin made during a local radio interview last week.

But what seems to have really pushed the Democratic senator to intervene in a Republican primary was not his friendship with Mr. McKinley but his anger over Mr. Mooney’s opposition to the infrastructure bill.

“Mooney’s vote against the infrastructure bill shows he isn’t interested in what’s best for West Virginia,” Mr. Kott said.

In the radio interview, Mr. Manchin also took a swipe at “Maryland Mooney.”

“Alex came here, I think, for political opportunity. I can’t figure any other reason,” he told the radio host Hoppy Kercheval.

It is only one House seat in a very particular state, but the narrative of the West Virginia race has caught the attention of a wider audience trying to divine how firmly Mr. Trump has the Republican Party in his grip.

“I think we’ll all be watching the returns Tuesday night,” Mr. Widmeyer said, alluding to the author J.D. Vance’s come-from-behind victory in the Republican Senate primary in Ohio after Mr. Trump endorsed him. “This will be the second week where we’re watching the influence of Trump on one candidate.”