The top Senate Republican super PAC booked more than $67 million in fall television ads across six battleground states on Monday, an investment more than twice as large as the group's spending at the outset of the 2018 midterm elections.

The ad reservations by Senate Leadership Fund (SLF) include $21.8 million in North Carolina, $12.6 million in Iowa, $10.8 million in Kentucky, $9.2 million in Arizona, $7.2 million in Maine and $5.5 million in Colorado. Senate Republicans are fighting off well-funded Democratic challengers in all of those states.

The ads are expected to begin running the day after Labor Day.

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“Protecting our Republican Senate majority has become synonymous with ensuring a firewall against the Democrats’ far-left ambitions," SLF President Steven Law 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 ad reservations were first reported by Politico.

The Senate races in North Carolina, Arizona, Colorado and Maine are expected to be particularly competitive.

SLF’s nearly $22 million investment in North Carolina — the largest in any state — is intended to protect Sen. Thom Tillis Thomas (Thom) Roland TillisMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Trump supporters chant 'Fill that seat' at North Carolina rally MORE (R-N.C.), who will face off against former state Sen. Cal Cunningham (D) in November.

In Arizona, Sen. Martha McSally Martha Elizabeth McSallyMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Trump supporters chant 'Fill that seat' at North Carolina rally MORE (R) is expected to face retired astronaut Mark Kelly in the November general election. In Colorado, Sen. Cory Gardner Cory Scott GardnerMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 MORE (R-Colo.) is likely to face the state’s former governor, John Hickenlooper John HickenlooperGardner on court vacancy: Country needs to mourn Ginsburg 'before the politics begin' Vulnerable GOP incumbents embrace filling Supreme Court seat this year GOP campaign director: 'There's no doubt that Republicans will control the Senate' MORE (D). And in Maine, Sen. Susan Collins Susan Margaret CollinsMcConnell locks down key GOP votes in Supreme Court fight Most Americans think winner of election should pick next Supreme Court justice: poll Murkowski: Supreme Court nominee should not be taken up before election MORE (R) is fighting off a challenge from state House Speaker Sara Gideon (D) and a handful of other Democrats.

Democrats are hoping to capture control of the Senate in November after winning back a majority in the House in 2018. The party needs to win three or four Senate seats this year to gain a majority, depending on whether a Democrat wins the White House.