This bit in a Washington Post article about the challenges the Obama administration faces because of "ideologixal diversity" among Democrats spells it out pretty clearly:

"Maintaining a sense of common interest across the party is a paramount goal. Early on, administration officials and Democratic leaders agreed they would steer clear of controversial social issues such as abortion and same-sex marriage. And to the discontent of many liberal Democrats, Congress intends to remain generally silent on those fronts. 'They know the consequences of '94. It looms,' White House Chief of Staff Rahm Emanuel said of the legislative debacles in President Bill Clinton's early tenure that produced the 1994 Republican landslide. 'That division led to failure. . . . Our chances for success only come about by unity. That, as a culture up there, has been enforced by enough people that enough members believe.'" 

Of course, a lot of attitudes can change in five years and the culture is now way ahead of the White House on so many fronts.

(thanks, Pam)