The Republicans have long been able to enforce party discipline in the House and Senate by threatening and sometimes even carrying out primary challenges against incumbent politicians in their own party. That tactic has been a key element in causing the Republicans to consistently move towards a more extremist right-wing agenda over the last three decades. For reasons I do not clearly understand, the Democrats loath to conduct such exercises on their members, instead defending incumbents who could better by dispensed with. Sen. Joe Lieberman is one example of that.
Now Steve Benen together with Nate Silver makes the case that selected primary challenges are needed to make the Democratic Party in Congress more effective and more useful to the nation.
I'm inclined to agree. Diane Feinstein gets off the reservation too often, and California is a quite Democratic state now. She needs a primary challenge badly to either replace her or at least force her to represent her constituents.