The absence of these Democratic megadonors is contributing to a huge disparity in spending between pro-Republican and pro-Democratic groups. The groups wield huge influence in many House and Senate races because they can take in contributions of unlimited size.

In the last week, Republican-leaning groups outspent their Democratic counterparts on television by more than seven to one on Senate races and nearly four to one on House races across the country, according to data from the Campaign Media Analysis Group, which tracks political advertising. The gap shows few signs of abating, even with the midterm election season in full swing.

The donors’ reluctance stems from a variety of factors, including pessimism about the party’s prospects in November, but also President Obama’s strong condemnations of this kind of independent activity, both during the 2008 campaign and after he was elected.