A mathematical and material fallacy.
Declining to participate is just that, affecting the outcome exactly ZILCH. As I will do in California's governor race between two leftists, one a Republican and the other a Democrat. Those are the only two options on my ballot, and I am opposed to what both offer. Your calculation of it as a vote FOR the Democrat is pathetic seeing as how, as it was with Romney in 2012, a vote for the Republican is a vote for validating the Democrat agenda.
The only way to vote for a leftist is to cast your ballot for one. All other ways are purely imaginary.
For a site supposedly filled with supposedly intelligent thinking people, there are a bunch that need remedial civics badly.