... Today, almost all Democrats are to the left of all Republicans. The result is that the system behaves very differently when one party is in control than when they share.
So differently, in fact, that you can fairly say that the country has one Constitution with two distinct modes of operation.
In Mode 1 — unified government — the minority party in Washington, shut out of power, has every incentive to make the majority’s life difficult, and does so. Its partisans, with no stake in whether anything gets done in Washington, [DCT comment: and worried about drawing a primary challenge in the next election cycle from their party's radical wing] treat the government as if it were under control of an invading army.
The majority, lacking support from the out-of-power party, must govern on its own, which requires holding on to every element of its coalition, which means governing from the center of its party instead of the center of the country.
In other words, Democrats, when in total control, have little choice but to govern from the left. Republicans, who are even more conservative than Democrats are liberal, govern from the right. Policy is driven toward the edges, instead of the middle. ...
In Mode 2 — divided government — the dynamic is reversed. Both parties, responsible for governing, have a stake in success. Forced to negotiate and compromise, they drag policy toward the center, allowing moderates to feel represented instead of ignored.
Most important, the country itself becomes more governable and meaningful laws stand a likelier chance of passage, because neither side can easily blame the other for whatever is wrong and because any major legislation needs support from both parties to pass.
Jonathan Rauch, Divided We Thrive, NY Times, Nov. 6, 2010 (emphasis and extra paragraphing added).