“Protecting our Republican Senate majority has become synonymous with ensuring a firewall against the Democrats’ far-left ambitions," Steven Law, the president of SLF, said in a statement. "This initial investment won’t be our last, but it is more than double what we initially reserved in 2018, and it demonstrates we intend to hold the line.”

The largest portion of the investment is in North Carolina, where SLF has booked $21.8 million in the race between first-term GOP Sen. Thom Tillis and Democrat Cal Cunningham. The state is likely to be among the most expensive in the country this fall, given its status as a presidential battleground that also includes competitive Senate, gubernatorial and House races. Democrats expended significant resources to boost Cunningham through the March primary, and SLF also spent nearly $3 million meddling in the primary on positive ads for his failed Democratic opponent.

Iowa is the second-greatest investment, with $12.6 million booked in the race featuring first-term Republican Sen. Joni Ernst. Democrats have rallied around businesswoman Theresa Greenfield, though she faces several other candidates in a June primary. National Democrats have already been on the airwaves boosting Greenfield's image in the state.

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They are also spending in Arizona and Kentucky using state-based affiliates through Defend Arizona, which was active in the 2018 Senate race, and Keep Kentucky Great, a new organization created last week, according to a recent filing with the Federal Election Commission. Keep Kentucky Great will spend $10.8 million as McConnell, the majority leader, runs seeking a seventh term.

Defend Arizona will spend $9.2 million in the special election between GOP Sen. Martha McSally, who was appointed to the Senate after losing in 2018, and Democrat Mark Kelly, the best-funded candidate in the country.

In Maine, where Sen. Susan Collins is facing the toughest reelection of her career, SLF booked $7.2 million for the fall. And in Colorado, Democrats' top pickup target, SLF booked $5.5 million to defend first-term GOP Sen. Cory Gardner.

Senate Majority PAC, Democrats' top outside group, has not yet booked its fall advertising, but has already been active in several Senate races. Nonprofit organizations affiliated with both parties' super PACs have also already invested millions in these races and other battleground states, underscoring just how expensive the fight for the Senate majority will be.