If not for candidacies like Nader's, competing with the Democrats on the left, the Democrats would compete with the Republicans on turf farther to the right. Sure, in theory, we can talk about the lesser of two evils, but where do you draw the line? If you are willing to vote to re-elect a Democrat who runs concentration camps and assassinates U.S. citizens, then (to paraphrase Malcolm X) all the capitalists have to do is show you the Republican wolf to get you to run straight into the arms of the Democratic fox.
In 1844, James Birney, the Liberty Party candidate, running on an antislavery platform, cost Henry Clay, a candidate with a moderate position on slavery, enough votes in New York state to cost him the Electoral College and the presidential election, which instead went to James K. Polk. Polk, one of the worst presidents of all, invaded Mexico to get additional land opened up for slavery to expand into. So if you look at it from the narrowest point of view, Birney's candidacy was a self-defeating failure for the abolitionist cause.
However, there's more to the story. Agitation on the slavery issue led an ambitious Senator from Polk's own party to propose the Wilmot Proviso banning the extension of slavery into conquered territory. Wilmot and others then organized the Free Soil Party which elected 16 people to Congress and forced many Northern Democrats and Whigs to declare themselves forthrightly for or against slavery. Free Soilers then became part of the founding nucleus of the Republican Party that elected Lincoln and ultimately did away with slavery and made Blacks citizens.
Whereas if antislavery folks had confined their electoral work to support for Henry Clay as the lesser of two evils, the Democrats would have maintained their role as the main national party of the planter class, while the Whigs would've continued seeking any unprincipled compromises with the slave power that gave them a shot at political success.