From The New York Times: http://www.nytimes.com/2015/10/07/opinion/how-did-the-democrats-become-favorites-of-the-rich.html
Voters on both the left and the right often claim that there is no difference between the Democratic and Republican Parties, and of course that isn’t true. There’s a big difference between Elena Kagan and Antonin Scalia, for one thing. But there may be more to this argument than you think.
Democrats now depend as much on affluent voters as on low-income voters. Democrats represent a majority of the richest congressional districts, and the party’s elected officials are more responsive to the policy agenda of the well-to-do than to average voters. The party and its candidates have come to rely on the elite 0.01 percent of the voting age population for a quarter of their financial backing and on large donors for another quarter.
The gulf between the two parties on socially fraught issues like abortion, immigration, same-sex marriage and voting rights remains vast. On economic issues, however, the Democratic Party has inched closer to the policy positions of conservatives, stepping back from championing the needs of working men and women, of the unemployed and of the so-called underclass.
In this respect, the Democratic Party and its elected officials have come to resemble their Republican counterparts far more than the public focus on polarization would lead you to expect. The current popularity of Bernie Sanders and his presidential candidacy notwithstanding, the mainstream of the Democratic Party supports centrist positions ranging from expanded free trade to stricter control of the government budget to time limits on welfare for the poor.
“Both Republicans and many Democrats have experienced an ideological shift toward acceptance of a form of free market capitalism which, among other characteristics, offers less support for government provision of transfers, lower marginal tax rates for those with high incomes, and deregulation of a number of industries,” the political scientists Adam Bonica, Nolan McCarty, Keith Poole and Howard Rosenthal write in a 2014 essay titled “Why Hasn’t Democracy Slowed Rising Inequality?”