Perhaps I've mentioned this before.
If you've never actually engaged in political action, if you've never volunteered for or worked for a party, you may be under the impression that the two parties are both monolithic blocs, that all Democrats believe "what Democrats believe" and that all Republicans believe "what Republicans believe." It's not so, and it never was so. Both parties are coalitions of smaller parties, smaller interest groups, united in opposition to each other. There's enough cultural overlap, enough values overlap, between the constituencies in each party that it's hard to draw the exact borders between the constituencies, and thus count them, but even the naked eye can see the difference between a Country Club Republican and a Social Conservative and a Nativist (racist isolationist), even the naked eye can see the difference between a Progressive Democrat and a Civil Rights Activist and a Blue Dog Democrat.
After a very short time of being politically active, there's a term I came up with to describe tiny little constituencies that bring just enough volunteers or money to the table to be given lip service to, but not enough volunteers or money to the table to dominate even the smallest part of the party, either party. These are constituencies that the people who run the party consider "outside of the mainstream," as "fringe" or "extremist" constituencies; they perceive that the cost of giving their "fringe" anything that they ask for as a condition of their support for the party, a cost measured in fear of alienating "mainstream" and "swing" voters, greater than the cost of making promises to their tiniest constituencies and then breaking those promises.
The term I uses for this is a "fuck you" constituency. As in, "fuck you, Nativists, what are you going to do if we break our promises to you and grant amnesty to Mexican illegal immigrants? What are you going to do about it, vote for the Civil Rights Democrats?" As in, "fuck you, Progressives, what are you going to do if we increase corporate welfare spending instead of breaking up the giant monopolies, if we cut taxes on the rich instead of bringing back truly progressive income tax rates, if we rule out single-payer before the negotiations even begin? What are you going to do about it, vote for the Country Club Republicans?" As in, "fuck you, Social Conservatives, what are you going to do if we promise you prayer in schools and outlawed abortion and lift not a finger on it when we're in power? vote for the Lifestyle Liberal Democrats?" As in, "fuck you, Civil Rights Activists, what are you going to do if we promise to help the Black community but instead continue locking up black men at 9 times the rate we lock up other drug users, what are you going to do if we continue helping employers abandon and banks foreclose in neighborhoods with over 50% unemployment? What are you going to do about it, vote for the Nativist Republicans?"
And your average American, having been taught by the talking heads on television that the Blue Dog Democrats and Country Club Republicans are the only serious grown-ups in American politics, the only people who actually understand how the world works and that the other constituencies, like the Progressives and the Reformers and the Civil Rights Activists and the Social Conservatives and the Nativists are all immature, ignorant, and/or dangerous radicals, likes this system just fine. They nod with satisfaction as the Ivy League educated, corporate funded Blue Dogs and Country Clubbers maintain the fiction that their way of looking at the world is the only valid one, and ridicule the other candidates out of the primaries, narrowing both parties down to a solid, reliable core of pro-corporate candidates in favor of maximum military intervention. They still differ from each other as to which corporations should be taxpayer supported and funded, the liberal ones or the conservative ones, and they differ as to which wars the US should make an excuse to join in on, the wars that serve liberal causes or the wars that serve conservative causes, and those differences matter a great deal to the Blue Dogs and to the Country Clubbers.
That leaves your "fringe" constituencies with only two plausible options. They can stay in the party, donate their money, show up to volunteer, work the phone banks and the Get Out The Vote efforts, and vote on election day, while continuing to make the case, inside their parties and to the broader American electorate, that they are right, no matter what the Ivy League universities teach, no matter what the corporate media selected talking heads say. Or they can throw the race to the other side, to punish their own side for lying to them, by closing their wallets and staying home. Those are the same two choices, whether you're a TEA Partier who isn't on Dick Armey's payroll or you're a Progressive: let them keep lying to you and hope that some day they come to realize that you're right, or punish your own side for lying to you.