Sheldon and Miriam Adelson in Macau, China, on Sept. 13. (Anthony Kwan/Bloomberg News)

The fight for control of Congress is about to get louder and more expensive.

A late infusion of big money is allowing congressional super PACs to ratchet up their spending in the final weeks before Election Day, financing an air war that will inundate voters in key states, according to campaign finance reports filed Tuesday.

On the right, Las Vegas Sands chief executive Sheldon Adelson and other wealthy Republican contributors swelled the coffers of the Senate Leadership Fund, a GOP-allied super PAC that raised more in August than it had in the entire 2016 election cycle.

The super PAC, part of a suite of groups affiliated with American Crossroads, collected $28.2 million last month, surpassing the nearly $23 million it had raised in the previous 19 months. The vast majority of the group’s haul came from Adelson and his wife, Miriam, who gave the group $20 million. Chicago hedge fund executive Kenneth Griffin contributed $2 million, while New York hedge fund manager Paul Singer gave $1 million. An allied nonprofit group, One Nation, which does not have to disclose its backers, took in $14 million in August, officials said.

On the left, the pro-Democratic Senate Majority PAC raised $11.6 million in August, the group’s largest monthly take this cycle. The biggest donor was Alexander Soros, a son of investor George Soros, who contributed $1.25 million.

See where the 2016 Senate races stand
Democrats have a good chance to take control of the Senate just two years after losing it.

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The surge of funds means that areas with competitive congressional races will be swamped with commercials in the last seven weeks of the 2016 cycle, and it comes as Republicans have improved their chances of holding the Senate.

The energy is particularly intense on the GOP side, fueled in part by donors steering clear of this year’s contentious presidential contest. Republican-allied groups aired 23,905 Senate ads between Aug. 19 and Sept. 15, while pro-Democratic groups ran 14,370, according to an analysis of Kantar Media-CMAG data released Tuesday by the Wesleyan Media Project.

The spending imbalance appears likely to continue. The Senate Leadership Fund reserved $60 million worth of airtime in seven states between Sept. 2 and Nov. 8, while its Democratic counterpart has $32 million worth of time in five states blocked for the remainder of the race.

Shripal Shah, a spokesman for the Senate Majority PAC, said the imbalance shows that Democrats “are on offense across an expanding map and if the election were held today we would take back the Senate.”

“Even Republicans recognize this reality — that’s why their special-interest backers are spending so heavily to try and shore up vulnerable incumbents,” Shah said. “As long as our donors step up to the plate to counter Republican spending in the coming weeks, we’ll be strongly positioned come Election Day.”

Even as the air battle intensifies, some strategists question the value of the late ad blitzes, a dominant feature of the past several election cycles. “It seems like we haven’t learned all the lessons of 2012 and 2014,” said Zac Moffatt, a Republican digital-media strategist. “I would argue that dollars spent on getting out the vote through door-knocking, phone banks and digital is probably a higher return on investment. But when donors behave this way and give late, you almost box the entities into” running ads.

It was wariness about the value of super-PAC spending that led major donors such as the Adelsons to hold back from giving large sums early in this year’s cycle. The billionaires topped the mega-donor charts in the 2012 campaign cycle, with nearly $93 million in disclosed contributions, most of which went to super PACs backing the unsuccessful presidential bids of former House speaker Newt Gingrich (R-Ga.) and former Massachusetts governor Mitt Romney (R).

Going into this year’s election, the Adelsons indicated that they were going to be much more careful in how they distributed their funds. As of July, they had contributed $1.4 million to an assortment of GOP candidates and party committees but had yet to support any super PAC, finance records show.

That changed last month. Along with $20 million for the GOP Senate super PAC, the casino magnate and his wife, a physician, gave $20 million to the Congressional Leadership Fund, which backs GOP House candidates, as CNN first reported, and they plan to contribute $5 million to Future45, a super PAC opposed to Democratic presidential nominee Hillary Clinton. That group will also get at least $1 million from TD Ameritrade founder J. Joe Ricketts, as the Wall Street Journal first reported.

Sheldon Adelson also helped fill a fundraising shortfall for the Republican National Convention host committee, contributing $1.5 million in August toward financing the festivities in Cleveland, a new disclosure shows.

The hefty donations propel the Adelsons to the top of this year’s mega-donor rankings. But it remains to be seen how much of their funds will bolster GOP presidential nominee Donald Trump, despite Sheldon Adelson’s reported promise to spend up to $100 million on his behalf.

The couple made their support for Trump official during the GOP convention, posing for a photo with him. Their political adviser, Andy Abboud, tweeted the photo with the caption, “The Adelson’s with their choice for President!”

As of July, neither had donated to Trump’s campaign.