Robert Wright has a thoughtful response to those who refuse to support the “lesser evil” in an election.
An election is not the same as an opinion poll. It is where an actual decision gets made, and failure to select the least offensive winnable option is completely foolish unless it substantially increases the chances of a better option becoming electable in the near future. Opinion polls do provide a way of gauging the electability of various candidates and it may be important not to hide disapproval of actions and positions that we find offensive both to “send a message” and to show support for the preferred candidate in the hope that seeing such support will encourage others. But using the election itself to “send a message” of non-support to the lesser evil just has the cost of ceding the victory to an even worse alternative and so doing real (and possibly irreversible) harm for the duration of the upcoming mandate.