When is it ethical to vote for ‘the lesser of two evils’?

Robert Simpson in Aeon:

Suppose you believe the state should look after the wellbeing of the poor and combat the structural forces that enrich the wealthy. Suppose you’re in a two-party electoral system, and that the party notionally aligned with your ideals made a Faustian pact with business elites to shore up the policies that perpetuate poverty – low minimum wages, tax incentives for rent-seekers, privatisation of public services, etc. What kind of ballot should you cast? You can’t vote for the party pushing things further to the Right. And if you don’t vote, or you vote for someone who’s almost certain not to win, you’re helping that same regressive party get elected. Yet lending your support to the ‘lesser of two evils’ candidate, whose platform you don’t really support, feels like an unacceptable compromise to your ideals.

The moral dilemma behind these scenarios is the subject of a well-known argument in moral philosophy. Bernard Williams argued that you should care about maintaining integrity in your personal ideals – not necessarily at all costs, but at least a bit. That’s because you have a special proprietary responsibility for acts you perform. Those choices and acts are, in some special sense, yours, distinct from outcomes that result from combining your choices and acts with everyone else’s.

More here.