Some key wells for GOP fundraising seem to have dried up since the 2008 presidential election.  This map (via Politico) shows just how much:

So what's driving the decline?

Some of the most affluent communities across the country aren’t donating as generously to Republican presidential candidates as they did four years ago, a function of the recession — which struck between election cycles — and a primary schedule that pushed back the dates of several key donor states.

The GOP race’s volatility also depressed the numbers, fundraisers said, which don’t include giving to super PACs, the unlimited money vehicles that have reshaped American politics. In general, itemized contributions to Republican presidential candidates stand 30 percent below 2008 levels.

These are valid reasons, but I would also venture to add the Tea Party to the mix.  The grassroots conservative movement wasn't around during the 2008 election, but since its rise, distrust in the Republican Party has been on the rise, leading Tea Party supporters to funnel their money away from the party establishment.