To change the trend of nominating only the most extreme candidates voter behavior must change. This year the overall turnout rate for primaries and caucuses is around 11%, and the composition of that 11% is not an accurate sample of the overall population or even of the Republican Party. These voters are more partisan than the average voter. If more moderates showed up to vote in primaries and caucuses candidates couldn’t just court the base, they would have to appeal to moderate voters as well.
The objection to my claim that moderates should show up in greater numbers is that to vote in closed primaries people have to identify with the party holding the primary. Many moderates identify as Independents and are therefore excluded from the process. So even if they wanted to vote they couldn’t. But, the increase in people identifying themselves as Independents is an illusion, for Independents don’t vote for Independent or third party candidates, most of the time they vote either Democrat or Republican. Claiming to be Independent and then acting like a party loyalist in the general election is disingenuous. Independents could help produce moderate candidates—as most independents do hold more centrist views—if they registered with the party they most commonly voted with and turned out for the primaries. And in states with open primaries like Tennessee and Vermont, where independents can vote, they must to turn out at the same rate as party loyalists if they truly want a moderate candidate.
Or, Independents could actually vote in the manner in which they identify themselves to pollsters (gasp), that is, vote for someone not from one of the two major parties. Most people don't vote for a third party candidate because they feel that if a candidate does not have a legitimate shot to win then their vote is meaningless. First, your vote is meaningless. The chance that a single vote, your vote, will be the deciding one in any election approximates zero. Second, if you don't vote for the person who you think is best equipped to run the country--regardless of party--you are wasting your vote.
If people really are dissatisfied with the direction of American politics in general, and polarization in particular, there is one simple solution: Get involved in a sincere and meaningful way.