As an undecided voter

[BumRushDaShow]

29. Well... Seems the latest "thing"

which has been tested in both California and Maine, is "ranked choice". There you have (in those cases) "all" the candidates on the list and the one(s) with the largest totals prevail(s). For those states, I believe they had candidates of all parties on the ballot and in this "single primary" suggestion, it could just be all the Democratic candidates.

Doing that might also be an interesting way to automatically select both a Presidential AND a Vice-Presidential nominee - i.e., the #1 & #2 vote-getters. If the #2 declines, you could go down the list to the next highest. That way you could help to keep a lower count candidate's constituency for the ticket because they would have a stake.

Just throwing that out there as an off-the-top-of-my-head thought!