THE JUST-COMPLETED midterm elections have provided a fresh glimpse of the growing power of outside spending in support of traditional campaigns, and the “dark money” for which there is no record of the donors. Both trends threaten to undermine democracy and deliver an outsize role in the public’s business to special interests and super-wealthy contributors.

Much of the political action this year centered on a few competitive races, especially in the Senate. In a report this week, the Brennan Center for Justice examined outside spending in 11 races that were considered the most hard-fought. Not surprisingly, this is where the big bets were placed by monied interests. In these races, excluding the two political parties, groups that hide some or all of their donors accounted for an astounding 71 percent of the outside spending in support of the 10 winning candidates. (The 11th race, for a Senate seat in Louisiana, is headed to a runoff.) The dark-money groups are mostly those claiming they are tax-exempt social welfare organizations, which do not have to identify their donors. Thus, when the Senate comes under the control of the Republicans in January, the GOP will owe its success in part to the power of hidden fat cats.

A separate analysis by the Center for Responsive Politics shows that outside groups exceeded the spending by actual campaigns in a quarter of Senate races this cycle. This suggests that the hidden donors and big-money men are reaching a tipping point where they overshadow the candidates they support. Outside spending groups — the super PACs that must reveal their donors to the Federal Election Commission and the dark-money social-welfare groups that do not disclose donors — are increasingly carrying the load of election campaigns by paying for ads, getting out the vote and conducting opposition research and other activities that have usually been up to candidates. It’s an ominous and unwelcome trend.

Much but not all of this is coming about because of the Supreme Court’s 2010 decision in the Citizens United case to open the door to unlimited political spending by corporations, unions and others. Although outside spending began before the court ruled, the decision released a flood. According to the Brennan Center study, outside spending for all 33 Senate races in the 2012 cycle, not including the political parties, was $259 million. This time, outside spending not including the parties in the 11 most competitive Senate races was $342 million.

The first principle of American electoral democracy is that voters choose candidates based on what the candidates say and what they will do. No less an authority than the Supreme Court declared, in the Citizens United opinion delivered by Justice Anthony Kennedy, that “transparency enables the electorate to make informed decisions and give proper weight to different speakers and messages.” Unfortunately, the objective of transparency that Mr. Kennedy envisioned is less true today than when the decision was delivered. The nation is moving in the opposite direction, toward a campaign finance system dominated by dark lords of hidden cash.