AFP Action, a major conservative advocacy group backed by billionaire Charles Koch, is rolling out over $4 million worth of digital ads starting Monday backing Republican congressional candidates throughout the country.

The ad buy, first disclosed to Reuters, will buoy Republican Senate candidates in Pennsylvania, Wisconsin, Ohio, Montana and Nevada. In all of those states, Republican challengers are locked in relatively tight races with incumbent Democratic senators.

While Democrats currently hold a 51-49 Senate advantage, they will be defending several competitive seats when voters go to the polls on Nov. 5, and Republicans have a solid chance of flipping control of the chamber.

The ad buy, the group's biggest one-time purchase this cycle, will also support 11 Republicans vying for House seats in five states, including battlegrounds Michigan, Arizona and Pennsylvania.

The group is spending $3.5 million for ads in Senate races and $549,500 on House races, with $1 million dedicated to supporting the candidacy of Pennsylvania Senate candidate David McCormick. The latest ads will run on multiple digital platforms, including streaming services.

So far this election cycle, the group has spent slightly over $15 million supporting Republican congressional candidates, excluding the latest buy.

"This ad buy is a continuation of our 'Firewall Strategy' to prevent one-party progressive rule in Washington," AFP Action said in a statement.

AFP Action has not traditionally thrown its weight behind presidential candidates. This cycle, however, the group supported Trump challenger Nikki Haley in the final months of the Republican nominating contests.

The group ultimately cut off Haley in February after she lost the primary in her home state of South Carolina, and said that it would focus on trying to win the Senate and defend the House, where Republicans hold a slim six-seat majority.

The content of the ads vary, though they largely attack Democrats for failing to contain inflation and secure the U.S.-Mexico border. Polls consistently show both issues to be major vulnerabilities for Democrats.