I don’t understand this political strategy in the long-run. If the left always unflinchingly votes for the leftmost candidate then the optimal strategy for the DNC is always to choose someone just 1% to the left of whoever the Republicans are running.

The trumpers aren’t strong because they always vote. They’re strong because everyone knows that, if Trump isn’t on the ballot, they won’t turn out to vote nearly as strongly.

Combine this with the fact that basically every business interest wants right-wing politics and you get the perpetual rightwards slide of the Democratic Party.