About $4.2 million of that was "dark money," funds that come from organizations that are not required by law to disclose their donors. Tester managed to win reelection by 4 percentage points even as a vulnerable Democrat running in a state that Republican Mitt Romney took with more than 55 percent of the vote.

Overall, $51 million was spent by candidates, party organizations and outside groups on the race, one that was considered by Republicans an important pickup opportunity on their path to regaining control of the Senate. According to ProPublica, that's twice what was spent on Tester's first race, in 2006.

$21 million of the amount spent on the race came from 64 outside groups, and the party committees spent $8.9 million. The millions went toward swaying fewer than 500,000 voters, likely one of the higher vote-per-dollar margins in the nation.