Some advocates of backing "the lesser evil" actually prioritize civil liberties and human rights even less than they themselves imagined.
My recent article, "Why I Refuse to Vote for Barack Obama," is one of dozens I've written in the last several years criticizing President Obama for violating civil liberties, expanding executive power, and waging a secretive drone war that presumes all unidentified males killed are "militants." Why did it receive more attention, by many orders of magnitude, than any of those articles? One significant reason is that partisan Democrats reliably pay attention to every issue that might impact Obama's chances at the ballot box -- and frequently ignore many important issues that won't. Write that the president is killing hundreds of innocent foreigners, or routinely spying without warrants on millions of innocent Americans, or setting the reckless precedent that one man can secretly order extrajudicial killings on his word alone, and relatively few people pay attention. Add the notion that those failures should cost Obama votes and perhaps a million people will read it! Scores of partisan Democrats responded using language much angrier than any they've ever marshaled against the problematic policies under discussion. The experience reinforced my belief that causes are best advanced by signalling to politicians and their partisans that specific behavior will end up costing them winnable votes.
That strategy can backfire for some. In 1992, George H.W. Bush lost his reelection attempt in part because he broke his "no-new-taxes pledge," causing parts of his own coalition to turn against him. Some suggested that he should face a primary challenger, others that Ross Perot might be preferable. These voters were among the reasons that Bill Clinton, a politician even worse on taxes by their lights, was able to win.