Wall Street billionaire Michael Bloomberg spent $4.3 million this week to help Democrat Harley Rouda in his effort to unseat 30-year GOP Rep. Dana Rohrabacher in their fight to represent much of coastal Orange County.

In the process, Bloomberg, a former Republican, helped make the 48th Congressional District the nation’s most expensive active House campaign in terms of outside spending, according to the Center for Responsive Politics.

More than $18.2 million has been spent on the race by interested partisan groups, nearly three quarters coming from Democrats. The race’s price tag swells to $29 million when including money from candidate committees.

The 48th is one of four districts touching Orange County that Democrats have targeted in their effort to flip 23 seats nationally and gain control of the U.S. House of Representatives. Most of those contests are predicted to have razor-thin margins.

Spurred by lofty stakes and the region’s pricey TV market, the cumulative political spending in those four races has shot sky-high: $108 million since the start of last year.

Raphael Sonenshein, executive director of Cal State Los Angeles’ Pat Brown Institute of Public Affairs, said it is unprecedented to see this level of spending on Southern California congressional races. He noted the current flow of money into the region’s House races reverses a long-time pattern in which politicians from out of state come to California to raise money for their campaigns.

“The races are so close, and the battle for the House is so close, that the stakes are almost infinite this year, and whichever team wins is going to change history,” Sonenshein said. “Given all that, the incentive to spend money is pretty high.”

Two years ago most of the House races in Orange County currently bringing in the most money weren’t competitive. Three of the four easily were held by GOP representatives, even as voters in those districts chose Hillary Clinton over Donald Trump for president.

Sonenshein noted the politics and demographics of the region are shifting in ways that make it easier for Democrats to compete for office.

“Orange County and even San Diego are changing so quickly, and that coincided with the accelerant known as Donald Trump,” he said.

Bloomberg’s donation to Rouda came through his political action committee, Independence USA PAC. With less than two weeks before the Nov. 6 election, the money is being used on TV ads in the district, with the PAC attacking Rohrabacher for calling climate change “a hoax.”

Environmental issues have featured prominently in the race, where some coastal cities are spending to fend off the effects of sea-level rise.

Rohrabacher acknowledges that earth might be warming, but says the change is part of natural climate cycles that humans aren’t causing. He has called the matter a distraction from more pressing issues, such as immigration. Rouda has called global warming “arguably the greatest issue facing humankind long term,” and promoted shifts to clean energy.

Bloomberg has said he would spend $100 million this mid-term election to try to flip the House to Democratic control, making him one of the left’s top two donors in the nation. In backing Rouda, Bloomberg — the former GOP mayor of New York — is supporting a candidate who, like himself, swapped political parties.

Bloomberg’s spending comes following a poll released Tuesday by Monmouth University, showing Rohrabacher having support from 50 percent of likely voters, with Rouda at 48 percent. Those results flip exactly opposite, with Rouda holding 50 percent, under a prediction model supposing high Democratic voter turnout. Other recent polls have shown the contest as a dead heat.

The two campaigns have opposite explanations for why so much money is coming into the race.

Rohrabacher’s campaign manager, Dale Neugebauer, said Democrats are set to outspend Republicans 6-to-1 in the race’s final two weeks because liberals understand that Rohrabacher is well positioned to keep his seat.

“Billionaires from San Francisco and New York City are pouring millions of dollars into this district to buy the election,” Neugebauer said. “All the money in the world won’t convince Orange County residents to abandon their principles and vote for a liberal left candidate like Harley Rouda.”

Rouda’s campaign sees the spending as evidence that voters want Rohrbacher ousted.

“It’s the most expensive race in the nation because patriotic Americans from coast to coast are appalled by Dana Rohrabacher,” said Rouda spokesman Jack d’Annibale.