Campaign spending has shot upward for many reasons. Fund raisers have become much more skilled at energizing both small and big donors. A polarized political system has heightened the stakes in elections. Incumbents in safe House seats often spend more than their challengers by a factor of 10, and the incumbent advantage in a safe Senate race is only slightly less. Republican candidates normally hold a fund-raising edge over Democrats in congressional campaigns. The size of that advantage in a given election depends on which party has the most incumbents and, therefore, controls each house of Congress. Control of a legislative body is worth a great deal to a party, quite literally. Senate races broke from the usual pattern even more dramatically. For political scientists, measuring the effects of campaign spending is a complicated task. Corporations and labor unions had long been prohibited from giving money directly from their own treasuries to political candidates.