The largest voice is the growing third party voice, namely libertarianism.
I used to be a card-carrying (literally) Libertarian. I drank the cool-aid too. It is the party of the future, and it always will be.
View it this way. . .
Presidential elections are typically won by 2-5 percentage points. Let's say, for example, a democrat wins the election with 49% of the vote and the Republican loses with 47% of the vote. Another 3% goes to libertarian and the remaining 1% goes to all others. In this scenario the libertarian vote now holds a lot of power. If Republicans would have worked harder to earn that vote, they could have won the election. The option now would be to either try to incorporate more libertarian views next time, or risk losing again.
I'll give you a real example. In 2000, there was a close Presidential race between one George Walker Bush of the Grand Old Party, and one Albert Arnold Gore, Jr. of the Democratic Party, and a less close race between these gentlemen and one Harry Edson Browne of the Libertarian Party, Ralph Nader of the Green Party, Pat Buchanan of the Reform Party, and some other gentlemen who no one knows. Because of the peculiarities of American Presidential elections, the race came down to the outcome of the race in Florida.
And the final vote came down to 2,912,790 for Bush, 2,912,253 for Gore, 97,488 for Nader, 17,484 for Buchanan, 16,415 for Browne, and several thousand more for each of several small parties that had no chance of winning.
Since the difference between Bush and Gore's vote totals was around 500, the voters for each of the five next parties in the final result, most notably for Nader, whose participation ensured a Bush presidency, had to ask themselves if the outcome was worth voting on principle. Sure, Gore lacked charisma, and he wasn't as ideologically pure as Nader, but I would bet that there aren't many who think the future Mr. Inconvenient Truth wouldn't have been worlds better than an oil tycoon. And the rational people who voted on principle for a third party have to admit that they could have used their votes in a much more useful way.
Third parties lose because they function as spoilers. They always will. In times of exceptional political disruption, they can very infrequently rise to power. But this is exceptionally rare, and most of the time, they just provide the chance for their larger enemies to win elections. The only way that they can actually wield the power that you speak of is to integrate themselves into the power structures of the main parties, and get their hands dirty trying to change those institutions from within. Which, to the purists, makes them appear compromised, but anyone who wields real political power will be.
Where on the flip side, all of you who automatically vote republican regardless give republican candidates little reason to listen to what you want or care about, being that they will get your vote regardless.
No dispute here. Though, Big Data has made this much more challenging. The main parties know how we vote, down to the block or the house, so they are very good at figuring out who they can sway and who they can't. The people who are really powerful politically are either in politics themselves, or they are independents.