Democratic operatives trying to raise money for expensive ad campaigns report that the wealthy liberals (and, to some extent, labor unions) who wrote huge checks to independent groups for advertising campaigns in the past three election cycles are sitting on their wallets.

Democrats attribute the stinginess to a variety of factors, including the lingering recession, the absence of a single unifying enemy such as former President George W. Bush and fatigue among the wealthy donors who wrote big checks during the past two election cycles.

There’s also a degree of disenchantment with the Obama political operation, which discouraged big-dollar independent activity when then-Sen. Barack Obama’s campaign was shattering fundraising records during the 2008 presidential campaign and has done little to revive it since.