Here is a link to the November 2012 exit poll data. The data shows that whereas only 1% of self-identified Democrats voted for a presidential candidate other than President Obama and Mitt Romney, and only 1% of self-identified Republicans voted for a presidential candidate other than Obama and Romney, 5% of the self-identified independent voters voted for another presidential candidate.
Because independent voters are the voters most likely to vote for minor party candidates, this data shows that a top-two system, which keeps minor party candidates off the November ballot, injures independent voters more than it hurts Republican and Democratic voters.
The data also shows 50% of independent voters voted for Romney, and only 45% of them voted for Obama.
The data shows that 38% of the voters self-identified as Democrats, 32% as Republicans, and 29% as independents.
The data shows that 3% of liberals, and 3% of moderates, voted for a minor party presidential candidate; but only 1% of conservatives voted for a minor party candidate.
The data shows that only 5% of the voters think foreign policy is the most important issue in the presidential election. But among those 5% of voters, 11% of them voted “other” for President.