D
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The difference is pretty significant, I think. 26% of American nonvoters support Democrats compared to 15% supporting the Republicans. That is a sizable difference!
http://nonvotersinamerica.com/wp-content/uploads/2015/12/Summary-Report-12-13-12.pdf
That's not that much of a bias, though. If you look at Gallup's party affiliation tracker, about 25% of voters identify as Republican, 35% Democrat, at any given time. Given that far more non-voters do no support any party or support a third party, those figures are not especially different - it's equivalent to 22.5% identifying as Republican compared to 37.5% identifying as Democrat if they had the same do not support/third party figures; the bias is about 2.5%.
Note that I'm not saying that non-voters are dead balanced between left and right. I'm saying that the voting composition of non-voters is almost the same as that of voters w.r.t. the main parties. As you can see above, this is broadly true of America too.