Factor by which election spending by outside groups -- those officially unaffiliated with candidates' campaigns or parties -- has increased since the U.S. Supreme Court's January 2010 Citizens United decision lifting political spending restrictions: more than 2

Amount that outside groups spent on U.S. Senate races in 2010: $220 million

In 2014: $486 million

Percent of competitive 2014 Senate races where outside groups outspent the candidates: 80

Percent of the total spent in 2014 directed at just 11 of the most competitive Senate races: 90

Percent of spending in those races that came from outside groups: 47

From the candidates themselves: 41

From political parties: 12

Total amount super PACs have spent on the three federal election cycles since 2010: $1 billion

Of the 10 highest spending super PACs in competitive Senate races, number that received less than 1 percent of their contributions from small donors: 8

Percent of the super PAC money spent in all federal elections since 2010 that came from just 195 contributors and their spouses: 60

Factor by which the average contribution to the top-spending super PAC, the Democratic-leaning Senate Majority PAC, exceeded the annual income of the average U.S. household: almost 2

Rank of North Carolina among the states where outside spending was highest: 1

Amount that outside groups spent on the Senate race in North Carolina: at least $80 million

Amount of the spending in the North Carolina race that was "dark money" for which outside groups don't have to disclose donors: $40 million

Of the outside spending in support of Republican Senate candidate Thom Tillis, who won the North Carolina race, percent that was dark money: 82

Since 2010, factor by which dark money in Senate races has increased: more than 2

Of the $1 billion outside groups have invested in Senate races since then, portion that was dark money: almost 1/2

Amount of dark money spent on Senate races last year alone: $226 million

Factor by which dark-money support for Republican candidates exceeded that for Democratic candidates: more than 2

(The numbers in this index are from "Election Spending 2014: Outside Spending in Senate Races Since Citizens United," released this week by the Brennan Center for Justice.)