The Democratic Party has typically been a coalition of out-groups -- white Southerners and big city Catholics in the 19th century. Today, the coalition includes blacks and to a lesser extent Hispanics, unmarried women, members of the Millennial generation, public employee union members and, most important, the group that demographer Joel Kotkin dubbed gentry liberals.
They don't always agree. Blacks tend to oppose same-sex marriage, while gentry liberals strongly favor it. Labor unions want the Keystone pipeline, while environmentalists want to kill it.
Other difficulties have surfaced. Democrats to their surprise find themselves on the defensive on Medicare, needing to explain why they took $716 billion from it and gave it to the still unpopular Obamacare.
Late in the game, Obama decided to rope in Bill Clinton to give a big convention speech Wednesday night. In effect, he's trying to suggest his second four years will look like Clinton's.
In the meantime, there is evidence that the Democratic Party brand, to use marketing term, is in trouble.
Pollster Scott Rasmussen periodically estimates party identification on the basis of thousands of robocall interviews of likely voters.
His findings have been uncannily close to the exit polls. In the last quarter of 2008, his party ID numbers were 41 percent Democratic, 33 percent Republican. The 2008 exit poll showed Democrats ahead in party ID by 39 to 32 percent.
In the third quarter of 2010, Rasmussen pegged party ID as tied at 35 percent. The 2010 exit poll showed it exactly the same.
The third quarter of 2012 is not yet over. But the Rasmussen party ID numbers for the second and third quarters combined are ominous for Obama's party: 34 percent Democratic, 36 percent Republican. Republicans are up only 3 percent from 2008, but Democrats are down 7 percent.
The partisan playing field has changed a lot since 2008. But Obama's policies have not. There has been no pivot. It will be interesting to see how loudly and often the delegates cheer, "Four more years!"