”President Trump just announced this morning his support for Evan Jenkins,” a Jenkins robocall declared, taking some liberties with the president’s tweet, which did not favor him over Mr. Morrisey. The recording began with the phrase “This is a Trump voter alert” as it sought to further align Mr. Jenkins with the president.

In a statement responding to Mr. Trump, Mr. Blankenship said the president “is a very busy man and he doesn’t know me and he doesn’t know how flawed my two main opponents are in this primary.” He added that the “establishment is misinforming him because they do not want me to be in the U.S. Senate and promote the president’s agenda.”

That Mr. Trump would step into the contest at all underscores how alarmed party officials are about the prospects of a Blankenship victory.

The president has felt burned by Republicans when he injected himself into other races only to see his preferred candidates fall short. A victory by Mr. Blankenship would be highly embarrassing to Mr. Trump, coming on the heels of his inability to lift party nominees in Pennsylvania this year and in Alabama in December.

But the race may be close enough that Mr. Trump’s last-minute intervention proves enough to derail Mr. Blankenship, a power play that would only underscore his clout with the party base.



White House aides and Senate Republicans have been discussing for the last week whether the president should comment on the primary, and agreed he should do so if it became clear Mr. Blankenship could win, three officials familiar with the deliberations said.

By Friday, when party officials received internal polling showing that Mr. Blankenship was still in the hunt, they ratcheted up pressure on Mr. Trump to speak out against the former coal executive, who was imprisoned until last year and remains on probation.

White House officials had already begun considering potential language for a tweet over the weekend. In a wide-ranging call with Mr. McConnell on Sunday that was initiated by Mr. Trump, the president indicated he was willing to weigh in on the race. The president’s aides said he may record an automated call of his own against Mr. Blankenship.