(CNN) Former New York Mayor Michael Bloomberg is giving Democratic midterm candidates a last-minute boost, adding another $10 million to his $100 million effort to help Democrats wrest control of Congress away from Republicans.

The majority of the new cash will fund an ad buy though Independence USA PAC, Bloomberg's super PAC. The ad will begin airing on television and online in eight cities -- Chicago, Seattle, Detroit, Kansas City, Miami, Orlando, Portland and Atlanta -- on Friday. The ad will also air on national cable outlets, including MSNBC and CNN.

The ad buy from Bloomberg, reported first by CNN, comes during what has become an active midterm cycle for a politician who is openly flirting with a 2020 run.

The ad slams Republicans for failing to deliver policies that benefit the middle class and suggests that the Democrats are more committed to cutting taxes for the middle class and ensuring health coverage for people with pre-existing conditions.

"In Washington, one party is calling the shots and the middle class isn't being heard," a narrator in the ad.

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