While I can't speak for the authors of the letter, my takeaway was that there are 2 issues:
1) Who I should vote for - both in terms of people and parties, and
2) Do I need to agree with everything they (or their party) says, and make sure my views align with every one of their positions.
I think #2 is especially important in today's polarized climate, where the pressure is to take sides on everything.