” lets get a US constitutional amendment to have all state, US Presidential candidate selection votes (primaries/caucus/magic 8 ball, whatever), to occur on the same day.”
You’ve seen six different front runners this election cycle. If everyone voted on one day you’d get the flavor of the day. Better to divide the states into five groups of ten and have five primary dates two or three weeks apart. Rotate the order of the groups every four years and all closed primaries.
You get the flavor of the day in the general,why not the primaries
All the ads would be nationwide hot and heavy for the weeks preceding and then you.vote. Any problems get ironed out at the convention