The two Democratic organizations have each spent close to $32 million. By comparison, the Republicans’ Senatorial Committee has spent about $16 million, and the biggest GOP Super PAC, American Crossroads, has spent about $7 million.

The Senate Majority PAC is central to the Democratic campaign strategy of running furious attack ads against GOP senatorial candidates. Democratic candidates entered their races with exceedingly low approval ratings, thanks to their ties to the Obama agenda. The Super PAC ads—which accuse Republicans of being everything from corrupt social-lunatics to women-haters—drag down GOP favorability ratings, tightening races.

Mr. Reid’s Koch attacks also appear to be depressing donor giving to GOP Super PACs that could run their own attack ads and even the odds. Super PACs have to disclose their donors. That hasn’t bothered Democratic billionaires like former New York Mayor Michael Bloomberg, climate activist Tom Steyer or hedge-fund titan James Simon —all of whom have given multimillion-dollar donations to Senate Majority PAC.