The vote- or seat-maximizing incentive for the Republican Party as a whole is to lay low, put forward no "Newt Gingrich" villain figure, and let Obama continue to take the blame for the ailing economy, while avoiding fights they can lose, because of the President's superior bully pulpit and media presence.
The incentive for individual Republican Congressmen is…different.
When it comes to marijuana legalization, I believe that the "anti-" forces will muster as many parental votes as they need to, to defeat it when they need to. The elasticity of supply is nearly infinite at relevant margins. Legalization may appear "close" for a long time, but in equilibrium it will not spread very far. The "no" votes will pop up as needed.