Mr. Trump’s insistence on leading the party “affects every member,” Mr. Zeidman said, as lawmakers and would-be elected officials jockey for a Trump endorsement that is as powerful in a Republican primary as it can be problematic in a general election. “He’s already proven that he wants to have a major say or keep control of the party, and he’s already shown every sign that he’s going to primary everybody that has not been supportive of him,” Mr. Zeidman said. “He complicates everything so much.”... “All Republican roads lead to Mar-a-Lago,” said Jason Miller, an adviser to Mr. Trump. “Trump is still the straw that stirs the news cycle. His influence will be central to every speech and story line this week.” Those who have trekked there to meet Mr. Trump in recent months include Sarah Huckabee Sanders, his former press secretary and a candidate for governor of Arkansas; Senator Rick Scott of Florida, the chairman of the National Republican Senatorial Committee; and Representative Kevin McCarthy, Republican of California and the House minority leader.