From Vox:

“If people thought it mattered that Democrats controlled the Senate, then they would run to be a Democrat in the Senate,” said James Wallner, a political scientist with the conservative think tank R Street.

“Running for the president is no longer running to be the leader of the party, it’s running to be yourself, and the party will follow you,” Wallner said. “For God’s sake, look at Donald Trump. He wasn’t even a Republican, and now he is the Republican Party.”