It is not clear how significant his endorsement will be for Mr. Balderson, who already won his primary and is now facing a Democrat, Danny O’Connor, on Aug. 7. For Mr. Kustoff — who is facing a primary challenge from George Flinn, a perennial candidate who has lost four previous congressional races but can afford to pour millions of dollars of his own money into his campaigns — it could be more valuable.

But it may ultimately be Mr. James who reaps the most benefit from the president’s support.

Mr. James, a combat veteran and businessman, is in an extremely close Republican primary with Sandy Pensler, also a businessman. Whoever wins will challenge Senator Debbie Stabenow, the Democratic incumbent, in November, and Mr. James and Mr. Pensler have worked hard to tie themselves to the president, knowing how influential his support has been for other candidates in this election cycle.

Mr. James quickly thanked Mr. Trump for his endorsement on Friday and said in a statement, “President Trump endorsed me because he knows that I have the experience, from the battlefield to the boardroom, to secure the borders, defend our Constitution and protect Michigan jobs.”