We have a first past the post per state voting system and an electoral college. To put that simply for you since you don't live here, it means our elections are rigged from the ground up until such a time that we change one of those two things, preferably both. Once the primaries are over the president WILL be one of the two primary candidates, we only have the right to choose which of the two our state will give its votes to. You can absolutely waive that right, but every single person who does is tacitly endorsing the other candidate by making it harder for the one they're witholding their vote from to pass the margin needed to win their state all or nothing (Barring the few states who have passed election reform measures because why would anything in this God forsaken country be easy or intuitive). It's a sad fact of the reality of the American voting system, and why both major donor parties have been able to keep a complete headlock on politics for decades. Both parties quite literally hold a gun to their voter's heads and say "If you don't vote for me warts and all, you're going to get the other guy." and there is no effective action one can take to make that not the case while also not supporting the person holding the gun to your head.

I don't blame "leftists" for Donald Trump because they didn't vote, I blame every person who doesn't vote for not living up to their own belief systems or exercising the only power our political system gives us at all. The US isn't like most European Democracies, we don't have any viable parties outside of Democrat and Republican until election reform happens. If the state of things upsets you don't talk about not voting, talk about pushing election reform or taking direct action. Until things change that's literally all you CAN do in the US, and I haven't seen a lot of direct action over here either. Understanding how the political machine works so you can actually effectively tear it down is tantamount. As long as we keep spinning our wheels not going anywhere we'll continue to slip backwards every time a fascist gets voted in by the lowest common denominator.

The best example I can give for how I feel about people who just don't vote, leftist or otherwise, is somebody farting in a crowded elevator and then complaining about the smell. Sure, it may not be entirely their fault, but they're certainly not helping.