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The fundraiser is part of an ongoing effort from Bloomberg to thank vulnerable senators who fought for gun control measures. 

His group, Mayors Against Illegal Guns, has run ads thanking Sen. Pat Toomey (R-Pa.) for his work on a bipartisan measure, along with Manchin, to expand background checks.

Tickets for the Manchin fundraiser range from $1,000 to $5,000. The Mountain State senator isn't up for reelection until 2016, but he's already become a target of gun-rights groups — and the issue will likely continue to dog him in his red-leaning state.

Bloomberg has also sought punitive responses to a number of "no" votes that gun-control advocates see as persuadable, including Sens. Kelly AyotteKelly AyotteRNC chair warns: Republicans who refused to back Trump offer 'cautionary tale' OPINION: Democracy will send ISIS to the same grave as communism Kelly Ayotte joins defense contractor's board of directors MORE (R-N.H.), Jeff FlakeJeff FlakeOPINION: Congress should censure Trump for his unfit conduct Club for Growth endorses Nicholson in Wisconsin GOP primary Immigration battlefield widens for Trump, GOP MORE (R-Ariz.) and Mark PryorMark PryorMedicaid rollback looms for GOP senators in 2020 Cotton pitches anti-Democrat message to SC delegation Ex-Sen. Kay Hagan joins lobby firm MORE (D-Ark.), against whom the group has run ads.