Michael Bloomberg, the former New York City mayor, was the biggest outside spender aiding Democratic House candidates. Cheryl Senter/Associated Press
A week before the Nov. 6 election, Kendra Horn, a Democratic House candidate in Oklahoma, received unexpected good news from her television consultant: Michael R. Bloomberg’s political action committee, a leading supporter of Democratic candidates, had purchased more than $400,000 in advertising on Oklahoma City television.
Still, Ms. Horn’s campaign did not realize just how big a boost it was receiving. The ad was not just a general get-out-the-vote message supporting Democrats, as they had assumed. Instead it attacked her Republican opponent, the incumbent Steve Russell, on his education record.
Ms. Horn’s campaign manager, Ward Curtin, said that once they saw the ad, “Obviously, we recognized it was a big deal.”
With just days to go before the election, and Ms. Horn fighting an uphill battle in a deeply conservative state, Mr. Bloomberg’s group had effectively doubled the television spending on her behalf, catching Mr. Russell flat-footed and unable to respond.
Ms. Horn, who had trailed in both a local poll and in Mr. Bloomberg’s internal polling, won by 3,338 votes.
Big donors — like the Adelsons, the Uihleins and the Koch brothers on the Republican side, and Tom Steyer and George Soros on the Democratic side — have become integral and influential players in every election cycle. But in this year’s midterm elections, Mr. Bloomberg, the former mayor of New York, emerged as a powerful and effective force, as well as the biggest outside spender promoting Democratic House candidates, according to disclosures filed with the Federal Election Commission.
Records filed so far show that organizations controlled and funded by Mr. Bloomberg spent more than $41 million on 24 House races, much of it on eye-catching ads rolled out on social media and broadcast on television in the crucial final days of the campaign.
And while it’s impossible to conclude that any one factor tipped the balance in a race, Mr. Bloomberg appears to have reaped the benefits of his millions in giving. Democrats won 21 of the 24 races he sought to influence. Of those, 12 had been considered either tossups or in Republican districts.
“The mission was to flip the House. Success or failure would be defined by that,” said Howard Wolfson, a senior adviser to Mr. Bloomberg.
Assessing the election outcome, Kevin McCarthy, the House Republican leader, cited Mr. Bloomberg’s spending as a significant factor. “Michael Bloomberg’s money went a long way. He defeated a lot of people by writing those $5 million checks,” Mr. McCarthy told CNBC.
Kyle Kondik, managing editor of Sabato’s Crystal Ball at the University of Virginia Center for Politics, credited Mr. Bloomberg’s operation for picking smart races.
Oklahoma City’s population is becoming younger and more diverse, a fact that helped propel a Democrat, Kendra Horn, to victory in a traditionally Republican House district. But the city’s changing demographics are only one part of the story.
“I don’t think you could say they were the difference between the Democrats winning and losing the majority,” Mr. Kondik said, “however I think you could say that Mr. Bloomberg and his late money may have made a difference in a few of the surprising results that helped pad the size of the Democratic majority.” He pointed to Ms. Horn’s campaign as an example.
Mr. Bloomberg, who first ran for New York mayor as a Republican but recently registered as a Democrat, is considering his own run for the White House in 2020. He is scheduled to appear in Iowa, the first presidential caucus state, on Tuesday to host a premiere of a documentary.
Mr. Bloomberg’s organization in many ways mirrors the Senate and House majority PACs that raise and spend money to support individual candidates, focusing on close…