The reason for Republicans to vote Democrat in the next presidential election is that Donald Trump is the likeliest Republican nominee. And a vote for Donald Trump is a vote for the dissolution of the Republican Party.
The two of us like to think of ourselves as progressives. But we have family who are Republicans (or, at least, distant acquaintances). So, we respect the fundamentals of a Republican capitalism: a self-reliant, debt-free republic protected by a military budget big enough to guard several dozen less ambitious democracies.
Granted, Trump already has managed to put a hole in the bottom of each of those boats, creating a society in which only billionaires can afford self-reliance; in which the republic continues to sink deeper into debt; in which the military, police and security services are given money instead of respect.