This is the tricky problem. Do you say “the ends justifies the means” and throw your vote to the lesser evil candidate that can win (like Obama) or do you vote on principle and try to build up a real alternative?
If there was a real alternative, then we all (on the left, moderates, independents, etc.) would have voted for that person. It’s notable that you didn’t name a real alternative. Who is this person in reality that can take out greater evil Republicans in a presidential race?
In the horrible, political realities of 2008 and 2012, the lesser evil that was allowed by the corporatists to run and continue to run was Obama. We won’t end up with a worthy lesser evil until after decades of consistently supporting lesser evil. It’s just the reality we live in especially after a near decade of pure regression from allowing greater evil to rule.
Voting for Obama was, indeed, a vote on principle and it remains so to this day. The alternative was worse and there wasn’t a viable third party candidate that had a chance in hell of winning. To vote for a guaranteed loser (or to not vote at all) would have only helped to usher in McCain/Palin or Romney/Ryan.
What wasn’t principled was all the people that threw away their votes on a third party (or didn’t vote at all) and helped to usher in GW Bush for two terms and all the lackey, rubber stampers in congress to boot.
It’s time to vote in lesser evil consistently for a couple of decades to lessen the damage that’s been done already.
Many wonder how Democrats became Republicans. They should look in the mirror. 
When the American public is inconsistent and votes in Republicans (either directly or indirectly), many Democratic politicians surmise they need to shift increasingly to the right in order to win seats. By not supporting lesser evil Democrats, we send a message loud and clear to others that they need to shift to the right in order to make a greater impact (to win or stay in office at all).
If we would have consistently voted in lesser evil Democrats all these years, even the Republican party would have shifted more the left by now. Instead, here we are with an infestation of DINOs and Blue Dog Democrats as the barely lesser evil and a Republican party that’s so far to the right, that they’re dangerously unhinged.
We’ve allowed greater evil to become entrenched in our government by our own inconsistency. And, because of this egregious mistake it’s now going to take (literally) decades of consistently voting in lesser evil before there’s even a small hope of a feasible third party candidate that has a chance in hell of winning; Especially when you consider how much the greater evil employs voter disenfranchisement among other tactics to stay in power.
If you know of a third party candidate and/or democrat that is a viable threat to the current power structure, then by all means tell us who that person is now. But, if in the end, that person can’t beat the greater evil, then I will most definitely vote for the lesser evil instead (even while still principally supporting this person financially until they gain enough strength down the road).
Who should we give our support and money to? Who can take on this current power structure and WIN? For the love of God, please step up now and let us all know your name.
Otherwise, the principled thing to do is to vote lesser evil. But, I really do talk about this too much already.
