A group backed by Charles and David Koch dropped $2 million into the Pennsylvania Senate race Tuesday, targeting Sen. Pat Toomey's Democratic challenger Katie McGinty.

Freedom Partners Action Fund announced a new ad buy Tuesday focusing on McGinty's past career as a green energy lobbyist, which has been a main attack line from GOP-backed groups and Toomey himself. Particularly, they charge that McGinty helped "steer" millions in taxpayer dollars to green energy groups, which in turn contributed to her campaigns and paid her as an adviser.

The ad campaign is set to run until the end of August, though the group declined to say where it would specifically air.

In Pennsylvania alone, the group has dumped $5.5 million into the race, which is a must have for Republicans as they look to keep hold of the Senate majority. Overall, Freedom Partners has been a major player in Senate races across the map, pouring about $20 million in states like Ohio, Indiana, Wisconsin and Nevada.

Notably, Freedom Partners, along with Americans for Prosperity and other Koch-backed groups, have thus far remained on the sidelines on the presidential race, having declined to support Republican presidential nominee Donald Trump. However, they have also refused to back Democratic nominee Hillary Clinton, leaving them to do their work on the congressional battlegrounds.

At the moment, the Pennsylvania race is virtually deadlocked, with McGinty leading by less than a percentage point according to the latest RealClearPolitics average.