Primaries aren’t true elections. There’s nothing in the Constitution governing them. If the parties wanted to change their rules and decide who gets the nomination by playing paper, scissors, rock then they could. Primaries are just a way for the party to decide who to fund and support in the general election.
The primary is going to happen regardless, too late to change the election (and the GOP Senate will never go along). And it will be after Iowa/NH, so all they’re doing is undermining the Michigan Democratic Party. That doesn’t seem to make much sense.
Nor are caucuses and conventions mentioned in the Constitution either. Nor are parties! Even the word “city” is not there.
Except for the fact that parties now have been given official standing in many states, and the primary process has been liewise co-opted by the government in many states.