I have previously explored the logical absurdity of insisting that those who don't vote for one of the two foremost candidates in an election are effectively voting for the other of these two candidates. That analysis could easily be generalized to include ballot measures; where abstaining from voting has sometimes been claimed to be the same as opposing a measure, and sometimes been claimed to be the same as supporting the measure.
The primary purpose of claims of these forms is to pressure someone into voting for a candidate whom — or measure which — the potential voter finds unappealing. We especially see these claims when there are actual candidates, or formulated ballot measures. But we also see that purpose at play when an election has been held and candidates and proposal for the next election are not yet identified. Some of those declaring
You are now to blame for X because you did not vote for Y! are hoping to motivate the audience to tolerate whatever is demanded by the claimant's faction in that next election. An alternative would be to promise to offer something better than Y in that next election; but they are engaged in brinksmanship, threatening to take the community off a cliff if a plurality don't agree to their demands.
Other motivations for such claims in the wake of an election are simple ventilation and blame-shifting. It is frustrating to lose an election, and a blow to the ego to acknowledge that one's own faction may be largely responsible for that loss.
There's another, unrecognized motivation for these claims. Although there is a very great logical distance between refusing to support one of the two major political tribes and thinking as do members of the other major tribe, it is easy enough for tribal members to disregard that distinction almost perfectly. Thus, these absurd claims that refusal to support Y is operationally the same thing as supporting X implicitly become part of a more general psychological device of treating politics as all falling along a left-right spectrum, and thereby avoiding any challenge to one's thought or behavior that cannot be dismissed from the left by pointing to the right or from the right by pointing to the left, and saying
You guys are worse!, even if the challenger is not one of those guys. The challenge may even be spuriously taken as proof that, after all, the challenger were really a member of the other tribe, as he or she is not challenging them, and them alone.
 For example, James Henry (
Jim) Webb could have beaten any of the Republican candidates for President in 2016, and it is at least plausible that Bernard (
Bernie) Sanders could have beaten Donald J[oseph] Trump; but the Democratic Party chose its weakest or second-weakest candidate. (Martin Joseph O'Malley might have been a still worse choice, as his mythology of Baltimore collapsed in the face of the police murder of Freddie Carlos Gray jr.)