But the president’s reelection campaign has been aggressive in races where Trump has explicitly endorsed one candidate over the other, even as some of his endorsed candidates have fallen short in recent nominating contests.

“Your campaign mailer is misleading. In it, you repeatedly reference President Trump by name, refer to yourself as ‘the pro-Trump conservative’ in the race, and even include a photograph designed to appear as if the President is approvingly pointing in your direction,” Trump campaign strategist Michael S. Glassner wrote in the letter.

By sending the letter, Trump has waded further into a Republican congressional runoff for a seat the party is in danger of losing to Democrats. He endorsed Gonzales in a July 3 tweet.

Gonzales, who narrowly beat Reyes for the top spot in the March 3 primary, is endorsed by the retiring incumbent, GOP Rep. Will Hurd, and is the preferred candidate of national Republicans, including House Minority Leader Kevin McCarthy and House Minority Whip Steve Scalise.

House GOP leaders were lobbying Trump to get behind Gonzales when Sen. Ted Cruz (R-Texas) intervened, turning the race into a proxy war for competing national interests.

Cruz had been thinking about the race for a while and was leaning toward endorsing Reyes, a retired Air Force lieutenant colonel. Cruz’s father had already announced his endorsement, and the firm of Cruz’s top strategist, Jeff Roe, was working for Reyes.

Cruz and Trump spoke by phone in late June. During the call, Cruz made the case to Trump that Gonzales, a Navy veteran who spent time in Sen. Marco Rubio's office as a Defense Department fellow, had been insufficiently supportive of the president.

But Trump was unmoved. He broke with the senator and sided with House leadership and endorsed Gonzales. Cruz publicly backed Reyes, and his leadership PAC launched a six-figure ad buy.

House Republicans are highly invested in the outcome of the primary. The seat is a perennial House battleground and one of just three Republican-held districts that Hillary Clinton carried in 2016. A GOP loss would set the party even further back in its uphill effort to reclaim the majority.

The vast West Texas district spans from the suburbs of San Antonio to the outskirts of El Paso and includes some 800 miles of the U.S.-Mexico border.

Hurd won by fewer than 1,000 votes in the midterms, and his challenger, Democrat Gina Ortiz Jones, is running again. Democrats are delighted by the fractured GOP field, which they say has given them a head start.

Jones had more than $3 million in her campaign bank account by the end of June, her campaign said.

Alex Isenstadt contributed to this report.