Washington (CNN) One of the Democratic Party's largest super PACs on Thursday announced a $100 million campaign to reach about 2.5 million voters in four swing states -- an early push by Democrats to challenge President Donald Trump in what will be a bruising and costly race for the White House in 2020.

The effort by Priorities USA starts now and focuses on expanding and mobilizing the pool of likely Democratic voters in Florida, Pennsylvania, Wisconsin and Michigan -- states that backed Trump in 2016 after supporting his Democratic predecessor, President Barack Obama, four years earlier.

The campaign will rely heavily on digital advertising and media outreach to link Trump and his administration to Washington inaction on issues such as soaring health care costs, the group's chairman Guy Cecil said Thursday.

About 80 staffers will work on the new effort, he said.

Priorities' spending blitz comes as a slew of Democrats compete in earnest for the party's nomination and Washington braces for completion of special counsel Robert Mueller's Russia investigation. The President has repeatedly called the probe a "witch hunt."

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