Billionaire Paul Singer was an early and generous backer of Mitt Romney in the 2012 campaign, investing $1 million in a super PAC supporting the Republican presidential nominee more than a year before the election.

In this year’s White House contest, however, the hedge fund magnate has made clear that he’s not putting any resources behind Donald Trump. Instead, Singer is spreading his money down the Republican ballot: He shelled out more than $7 million through the end of June to nine super PACs backing congressional candidates.

He’s in good company. Some of the country’s wealthiest Republican donors are targeting Senate and House races around the country, hoping a financial firewall will protect the party’s congressional majorities on Nov. 8. Their investments — fuel for a record haul by super PACs this year — reflect a fear prevalent throughout the party: that Trump’s contentious candidacy threatens to trigger an electoral rout up and down the ballot.

Those worries spilled into public view Thursday, when a letter signed by more than 75 longtime GOP officials and party veterans asking the Republican National Committee to shift its resources to vulnerable Senate and House candidates was made public.

A defense system is up and running in Ohio, where a super PAC supporting Sen. Rob Portman against Democrat Ted Strickland has attracted contributions from a who’s who list of heavyweight contributors. Along with Singer, who gave $750,000, the Fighting for Ohio Fund has been buoyed by ­six-figure checks from Chicago hedge fund manager Kenneth C. Griffin, Boston investor Seth Klarman and Florida-based home builder Dwight Schar.

The group’s swelling coffers have helped make Ohio’s the costliest Senate race of 2016 so far. Already, the state has been pounded with more than $32 million worth of ads by at least 17 super PACs, unions and other politically active groups, according to campaign finance records.

Keen donor interest in down-ballot races this year has helped drive nearly $1 billion into super PACs, which can take unlimited contributions from individuals and corporations. Together, more than 2,000 super PACs registered with the Federal Election Commission collected $937 million by the end of June, according to a Washington Post analysis of FEC data.

That haul exceeds the totals amassed by super PACs in every previous cycle since they came on the scene in 2010 — including the 2012 elections, when such groups raised $853 million by the end of the year, according to campaign finance filings.

Money is cascading not only into Senate races. Republican donors have taken notice of House Speaker Paul D. Ryan’s warnings that his chamber’s majority cannot be taken for granted, and they have flocked to his allied super PAC, which has seen a burst of first-time contributors, officials said.

[Paul Ryan is confident about keeping his seat, more cautious about the House]

“It’s a trend that’s been picking up momentum,” said Mike Shields, president of the Congressional Leadership Fund, which backs GOP House candidates. “We have more money in our super PAC at this point than we have ever had in a cycle.”

And many GOP contributors have immersed themselves in the dozen gubernatorial races taking place this fall, hoping to expand their party’s hold in the states. Fred Malek, finance chairman of the Republican Governors Association, said the group has added to its ranks of top-tier donors in recent months.

The number of influential Republican officials saying that they can't vote for either Donald Trump or Hillary Clinton is growing as Sen. Susan Collins (R-Maine) pledges she won't vote for Trump. (Jenny Starrs/The Washington Post)

“We’re having a high degree of interest,” he said. “The RGA is quite robust. We are the one organization that has pick-up opportunities.”

Nearly half of the money in this election cycle — about $420 million — has been spent on the presidential race, including the bruising primary contests. But a recent surge of contributions to groups focused on down-ballot races is poised to fuel costly Senate and House battles this fall.

[Meet the wealthy donors who are pouring millions into the 2016 elections]

Super PACs and politically active nonprofit groups that support congressional candidates have reported spending $142 million on voter outreach as of Wednesday, compared with $158 million at this point in 2014 and $81 million by this date in 2012, finance records show.

In the coming weeks, that figure is set to balloon. On the Senate side alone, super PACs and other outside groups have reserved nearly $100 million in airtime for state television buys through Election Day, according to a strategist tracking TV ads. Those early reservations are expected to be followed by an even larger wave in the final fall stretch, as GOP allies such as the U.S. Chamber of Commerce, the Senate Leadership Fund and the Koch network intensify their efforts to help Republicans hold the Senate.

One additional possible cost driver: an expanding battlefield. Strategists on both sides of the aisle are closely watching states such as North Carolina and Arizona, where a strong anti-Trump vote in African American and Latino communities could boost Demo­cratic challengers.

To counter that, Republicans are trying to separate their candidates from Trump, hoping that voters who back Democratic nominee Hillary Clinton will cross party lines further down the ballot. That phenomenon helped GOP Sen. Dean Heller hold on to to his seat in Nevada in 2012, even though the state went for President Obama.

“People understand that the top of the ticket has its own brand,” Shields said. “But having said that, any time you see numbers coming out where [the nominee is] down by double digits, you have to prepare yourself and gather the necessary resources to make the case for that separation.”

Trump campaign officials did not immediately respond to a request for comment.

Democrats, sensing an opening, are building their own financial arsenal. Senate Majority PAC, the super PAC that supports the party’s Senate candidates, has raised nearly $24 million — including $1 million from billionaire investor George Soros in June.

“We have what’s shaping up to be a potentially historic cycle to not only elect the first woman to the White House but also a Democratic Senate that can act on key priorities, starting with the ­Supreme Court vacancy,” said Shripal Shah, a spokesman for the super PAC. “That’s a message we’re sending to our supporters, and they’re responding.”

The high stakes have drawn early firepower to states such as Pennsylvania, where Senate Majority PAC and other Democratic allies are squaring off against the Koch-backed super PAC Freedom Partners and other supporters of incumbent GOP Sen. Patrick J. Toomey. The tally so far: $21 million and counting.

[Koch network seeks to defuse donor frustration over Trump rebuff]

There are signs of a big money fight brewing in Illinois, where Republican Sen. Mark Kirk is scrambling to fend off Democratic Rep. Tammy Duckworth, even after Kirk formally retracted his support of Trump.

Kirk’s plight has drawn the attention of major GOP financiers, who have seeded a super PAC supporting him with large checks. Independent Voice for Illinois collected one of its biggest donations to date in late June, when it received $150,000 from out of state.

The donor? Paul Singer.