If your big thing is opposition to taxes or opposition to abortion...

They came up with the anti-tax pledge and they have insisted all their candidates be anti-choice.

Over time the party came to reflect those values in a fairly solid manner.  The Club for Growth continues to do primary challenges to reinforce the tax thing.

On abortion, they have been getting their judges.  Do the Wall Street types want to outlaw abortion?  No.  But they can't control these folks anymore after ceding them so much power.  At many points anti-abortion folks could have given up after realizing that the power brokers were just using them and just paying lip service to their issues.  

But in the end they succeeded in taking over the party.  They've made more inroads in opposing abortion than they ever could have accomplished in a third party.  

I don't like to compare myself to the anti-abortion crowd but there is a lesson there.  You can change the party even as the power brokers within it are using you, paying you lip service, etc.  But you have to fight over the long-haul, not get discouraged...you have to be cynical without letting cynicism deflect you from your goals.  And eventually, the power brokers will cede you so much power that they can no longer control you.  At that point, the party has changed into something different.

If we want a different kind of foreign policy, if we want universal health care, if we want de-consolidation of the media, if we want our civil rights back, then we are going to have to break the consensus on these issues within the higher echelons of the Democratic Party.

Along the way we will have countless opportunities to be insulted by the party and be dismissed by the party... and the temptation is to walk away from the party.  

But there is no walking away from this two-party system.  You are either within it or you are on the sidelines.  

The only thing a third party can do is punish centrist thinking and try to teach one of the two parties to pander to their issues.  But they can't fundamentally alter the center because they have no power and cannot vote on or craft legislation.