The economic structure of American politics is often discussed in class terms, which are of course relevant, but there's a substantial sector-vs-sector basis as well. To pick an obvious point if you work in the wind turbine industry at any level you're better off with the Democrats in office and if you work in the oil extraction industry at any level you're better off with the GOP. Open Secrets' chart of contributions to Barack Obama and Mitt Romney by employer shows a similar sectoral divide.
Obama gets a fair amount of money from people working in the financial services sector simply because a lot of people with a lot of money work in that sector. But all five of Romney's top sources of money are financial services firms. Obama, by contrast, is getting a lot of his money from people who work in the higher education and high-tech sectors.
Wall Street's dominance of Romney's fundraising is particularly interesting because it's a new thing. Back in 2008, Obama got tons of Wall Street cash most likely on a similar basis to his current high-tech fundraising—it's an industry with a coastal liberal sensibility on cultural issues and no deep investment in the minimum wage or pollution regulation. But then bank regulation suddenly became a hot button topic, and the Obama administration pushed a Dodd-Frank law that Wall Street despises and that Republicans have promised to repeal and return us to the pre-crisis regulatory framework. The whole subject has really become a dog that didn't bark during the campaign, but the huge shift in campaign contributions underscores the fact that it's a significant issue.