Win some primaries. It's not hard to grasp. You think some Democrats aren't left enough? Beat 'em. Did someone you believed was a strong progressive end up voting or governing more to the center than the left? Back a primary challenger. Beat 'em. Knock out the Democrats you don't like and replace them with someone to their left. That's how you move the country left. Then work yourselves to the bone so that those progressives you helped win that primary beat their Republican opponent on Election Day. Because every election, whether the primary or the general, is about a choice. The way to advance progressive values is to back the most progressive candidate who can win. That's not always easy to figure out, but we have to try.
But you do not advance progressive values by electing Republicans. Ever. You don't "punish" Democrats when you replace them with Republicans. In doing so, however, you do punish their constituents—in particular the vulnerable among them—who lose their voice in fighting for their interests. And, going farther, if the Republican who gets elected provides the margin of victory on a bill that passes by one vote, well, then that means everyone in the city, or state, or country is affected by that election. Electing a Republican will not move the Democrats to the left.
So, let's review. Support the progressive candidate in Democratic primaries. In the general election, again, support the more progressive candidate. Almost always, this will be a Democrat in a contest against a Republican.
Here's the thing we also have to accept. If the progressive candidate loses in the primary, that means the more progressive ideas may not be as popular as we think they are among Democrats actually who vote. That sucks. It means we have to fight harder next time, after we vote for the better candidate in the general.
The reason why Tea Party candidates were able to win a bunch of primaries was—and this ain't rocket science—they got more votes than the less radical, establishment candidates. As I said above, they moved their party to the right not by withholding votes on Election Day, but by getting out their votes on Primary Day.
That's the lesson we progressives need to learn. And remember.