The Republicans fought like dogs to win the Florida recount in 2000.  Norm Coleman dragged out the election in Minnesota.  George W Bush passed two tax cuts via reconciliation in his first term.  These policies play to Republican partisans but alienate moderates and independents.  Wary of losing the votes of independents,  one loss in Massachusetts has left the Democratic Party reeling and ready to step back from healthcare reform.  Why are there differences between the parties in their focus on partisans vs independents?

Politicians are motivated both by ideology and reelection.  They must take both into account when taking a policy stance.  As party activists can influence the chances of reelection, they can affect the policy stance of the politician.  This logic holds true for both Democrats and Republicans but what differs is the risks extremists in the two parties are willing to take to influence policy.  Right wing activists are willing to decrease the probability of the Republican Party winning the election to increase the probability of having a policy closer to their ideal implemented should the party win.  They are willing to run their own candidate in the Republican primary and risk them losing the general election against the Democrats.  The recent congressional election in New York is an example of this. So, even moderate Republican politicians must take this threat into account and adopt more right wing policies to counteract it.

But left wing activists are not willing to take a similar gamble except in extreme circumstances (e.g. Ned Lamont vs. Lieberman in CT).  So, Democratic lawmakers can afford to woo moderates without losing the support of partisans.

This is part of the story but not all of it.  Most importantly, it relies on an asymmetry between the preferences of right wing vs left wing partisans.  A deeper theory would also explain the asymmetry.