Telling West Virginians “I can't help you anymore as a Democrat," Govenor Jim Justice of West Virginia announced his conversion to the Republican party during a Donald Trump rally.

The switch in allegiance was not quite a major ideological shift for Mr. Justice, a former coal industry executive who has at times been registered as a Democrat, Republican and independent.

Trump had previewed the proclamation earlier in the day while speaking to reporters, hinting at a “very big announcement.”

But the move still underscored Mr. Trump's powerful appeal in West Virginia, a state that he claimed decisively in the 2016 presidential election.

Paeans to struggling West Virginia coal miners were a common feature of Mr. Trump’s campaign, and the president returned to that theme early in the rally as he touted “putting coal miners back to work” by easing environmental restrictions.