…Part II…
One could argue I suppose that in an election one is choosing between perhaps bad options and one party or candidate might have something going for them, even if they also have some policies one doesn’t like one bit - they may have an immigration and housing policy that appeals strongly, even if they will cut health care spending and introduce a same sex marriage law (or whatever, I just made that up

). Or one may have personal respect for one’s local MP/Representative and one believes they do a lot of good locally, in which case even if one dislikes his/her party one might one to vote for them (maybe one could vote for a pro-choice Democrat in the US, because the amount of good they do locally exceeds the amount of harm nationally given that their party would support some hypothetical pro-choice initiative with or without their being in the House). If either of these (some good policies/good local candidate) is at all the case - I absolutely think one should vote, and it’s on this basis that I’ve voted in every national and local election since I was old enough. One might not feel this is the case.
However - one is not choosing between some bad options in a one-shot game. Because elections happen again and again every few years, one is choosing them in an ‘iterated’ or ongoing game. One’s choice
this time has a (minuscule) impact on the options available
next time.
So IF one cares about the tiny impact one’s choice has (say because making these choices contributes to social cohesion or improvement, as others in the thread have pointed out including Andreas who quoted §§2239-40 of the CCC) - then a long-run optimal position might still be to vote if no candidate is good enough, because it STILL influences what happens next time (hopefully being an incentive for more inspiring policies from one party which in future one can eagerly support).
One should, I believe, take a “you have a moral duty to do what will lead to best outcome for society if everyone did it” view of one’s actions. In the long run, withholding votes might be the best thing to do, because it would force would-be politicians to actually earn votes by promising and implementing good policy, rather than rely on the fact that we’ll vote for them because they aren’t as bad as someone else.
Here in the UK, the logic I’ve used would collapse if say UKIP were a more serious force in UK politics (I can’t think of an equally loopy but alarmingly popular party in Canada, sorry!). If one was in Germany in 1933, one probably wouldn’t feel that so little was at stake. Equally, and which is the same thing but backwards, if someone really,
really amazing who one believed would make the place much better, came along - either way the expected value of one vote would be higher, even if the likelihood of a single vote making a difference is much the same
Our guiding principle, at the end of the day, is to treat people with love. Except in the Hitler case (Godwin’s Law!) or in the Jesus-for-President case (absurd, I know), I don’t see that the act of voting is actually an automatic embodiment of that principle.
All that said, in the time I have spent constructing this argument, were today an election day in the UK, I could have made it to the polling station, voted, come home, and made a cup of tea, so frankly I think the default position for anyone in a democratic country like Canada or Great Britain would be to vote. If one doesn’t, it should only be because one actually cares about the outcome anyway.