Are there any rules or laws requiring the a party to hold primaries if they have an incumbent running for reelection?

Question

It is generally assumed that Donald Trump will be the republican nominee for U.S. President in the November 2020 election. However, from my understanding, he still has to actually win the primary; he is not just automatically the nominee. What makes him the assumed nominee is the fact that the vast majority of republicans voting in the primary are going to automatically vote for the incumbent if he is running again.

However, I have read that some states are not holding republican primaries. In these states, do the delegates just automatically go to Trump? Is that legal to simply skip a primary like this? Or, if not a matter of legality, is it within the rules that the republican party has set? Can the republican party simply say "we aren't having primaries; Trump will be the nominee and that's that"?

Finally, if there are many states where primaries will not be held, but delegates will automatically go to Trump; why would other states bother having primaries; when even if every single one of those states voted for Bill Weld it wouldn't make a difference?

Answer 1

In general, rules are for primaries are determined by each state party:

Franchise in a primary or caucus is governed by rules established by the state party, although the states may impose other regulations.

As for whether the action of cancelling a caucus is legal, the answer depends on the state rules for primaries.

Worth noting is that cancelling a primary for an incumbent isn't entirely uncommon. For example, the Democrats did the same thing in 2012:

Four states canceled their respective Democratic primaries altogether, citing Obama being the only candidate to qualify on their respective ballot: Connecticut, Delaware, New York, and Virginia.

Answer 2

It's up to party rules. A big reason to have a primary is that there are likely down-ballot offices which may still have challenges.