As Democrats and Republicans battled for dominance in both the House of Representatives and the Senate in 2018, super PACs and dark money groups collectively outspent the candidates’ own campaigns in a recordbreaking 16 races, according to data provided to Issue One by the Center for Responsive Politics. Control of the House and Senate for the next two years was determined by a small number of elections in 2018. Super PACs and dark money groups focused most of their spending in these races. Democrats needed a net gain of 23 seats to win a majority in the House and a net gain of two seats to win control of the Senate. Democrats ultimately flipped the House, but Republicans added one seat to their majority in the Senate. The new data shows that candidates were outspent by outside groups in seven Senate races and nine House races. This amounted to more than half of the 13 competitive Senate contests and about 10 percent of the House races ranked as competitive by the Cook Political Report. Non-candidate, non-party groups have now outspent candidates in 48 congressional races since the Supreme Court’s 2010 Citizens United decision, with one-third of that total occurring in 2018. This phenomenon occurred 11 times in 2016, and only once in 2010, according to the Center for Responsive Politics.