In racing there’s an old adage: “In order to finish first, you must first finish.”
The basic idea is you can’t drive so fast you fly off the circuit and take yourself out of contention. Priority #1 is you must finish the race since that is a prerequisite to winning it.
Like you, I’m sympathetic to the idea of a crossover ticket, as I suspect many here are. Harris / Romney, for example, sounds kinda awesome if you don’t bother thinking too deeply about it.
But the moment you look under the covers at what drives the voting numbers across districts and across electoral college states in the places that shift electoral outcomes, you can see pretty immediately that such a move would have one outcome: You would lose.
There’s a reason the political parties focus so heavily on particular blocs of voters and ideological camps, and it’s not because they’re seeking purity among their flocks (tho it surely appears that way sometimes).
It’s because that’s what it takes to win, and if you don’t win then it’s all just academic counting of how many angels can dance on a pinhead (no, I don’t mean Trump)