Every time the Dems lose, they go to the center to find voters. Want them to stop going to the center? Then give them overwhelming and consistent victories.
If you think you can change their platform by not voting or voting 3rd party, you're dead wrong. They will just go to the center voter even more. This is not a Mexican standoff that you can win, because they have an out which is worth double (a center voter is both a vote for them and vote taken away from the other party).