If the situation leading up to the 2015 general election was exactly the same apart from compulsory voting was introduced, what do you think the result would be?
Tories - 33
Labour - 32
UKIP - 17
Lib Dems - 7
Greens - 5
Worse for the Tories, a tiny bit better for Labour, considerably better for UKIP, more or less same for the Lib Dems, slightly better for the Greens. UKIP gain Thanet South, Thurrock, Boston and Skegness, hold Rochester and Strood, possibly gain Rotherham. Maybe even a couple more that I'm forgetting. Greens gain Bristol West. Galloway comes closer in Bradford West (though still loses). SNP still win 56 seats, possibly even adding the other three. Labour-SNP deal. Tories fall into civil war. Several defections to UKIP.
No democracy has compulsory voting. Some like Australia have compulsory *balloting*--not the same thing, since one can cast a blank or spoiled ballot without penalty. http://themonkeycage.org/2011/11/mo...ot-necessarily-make-electorate-less-informed/
Anyway, compulsory balloting does result in a higher percentage of valid ballots than non-compulsory balloting. This is supposed to help the Left on balance, though one may note it did not prevent victories by the Right in Australia in the federal elections of 1996, 1998, 2001, 2004, and 2013. In other words, it is definitely not a panacea for the Left.
Spoiling your ballot is voting. I think you're trying to make a distinction that doesn't exist.
If you want to call casting a blank ballot "voting," fine. The point is that those people who do not want to vote for any of the candidates do not have to do so, even under so-called mandatory voting. Therefore one cannot automatically *assume* that any party or candidate will get more votes than it got under non-compulsory voting. Of course in all likelihood there will be *some* people who otherwise would not have voted who would vote for one candidate or another, but not as many as one might think, and it is unlikely that they would vote ovewhelmingly for one candidate.
No democracy has compulsory voting. Some like Australia have compulsory *balloting*--not the same thing, since one can cast a blank or spoiled ballot without penalty. http://themonkeycage.org/2011/11/mo...ot-necessarily-make-electorate-less-informed/
....
But how many people are going to deliberately spoil their ballot? Very few.
So, ja, you're not going to get party vote adding to 100%, but there will be a lot more votes cast for parties than iOTL. Especially considering the ridiculously low turn outs. The 2014 US Federal election had a turnout of 36.3%, according to
http://www.nytimes.com/2014/11/12/opinion/the-worst-voter-turnout-in-72-years.html?_r=0