It's interesting. 99% of the time, the radical right on the court is ruling corruptly, following a radical ideology designed to overturn the constitution for the benefit of plutocrats.
But on presidential elections, they might be on stronger ground. Here's what the constitution says:
"Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress..."
In such manner as the Legislature thereof may direct. That means they can let citizens vote to decide. Or they can decide themselves. Or they can throw darts. In such manner as the Legislature may direct. And that language is the basis, I suspect, of the argument that the state constitutions and court have no say - it says right there, the LEGISLATURE may direct, so who can second guess them on state grounds?
We take things for granted, but that doesn't mean they're required. Before JFK, political parties picked nominees in 'smoke filled backrooms' and conventions. JFK saw that he'd lose that, and took advantage of rules allowing him to run in primaries across the country. In his private plane, he and Ted Sorensen flew to all 50 states before the primaries, and he won that way. Ever since, we take primary elections for granted.
There don't need to even be primary elections; that's a party choice. And the constitution says right there, while you get to vote on your state legislature, you don't get to vote on president unless that state legislature says you do. How many Americans know that? Republican lawyers sure do. And they have a plan.