Ms. Sanders may be a unique case study. She is stepping over both the Arkansas lieutenant governor and the state attorney general, who waited their turn and supported Mr. Trump but who can’t claim as close a connection to him as Ms. Sanders can. Mr. Trump was expected to endorse her on Monday night, an adviser to the former president said. She also has powerful family connections in the deep-red state, where the only race that really matters is the Republican primary.

“The Republican base is not interested in serious governing candidates,” said Tim Miller, who served as an adviser to former Gov. Jeb Bush of Florida. “They want people who will ‘own the libs,’ and Sarah is perfect for that.”

While a grievance-first strategy may be ruinous to the party’s chances in swing states like Pennsylvania and Virginia, it resonates in a conservative state like Arkansas. “Pro-coup Trumpism is a meal ticket to success in a Republican primary,” Mr. Miller said.