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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 AyotteOPINION: Democracy will send ISIS to the same grave as communism Kelly Ayotte joins defense contractor's board of directors Week ahead: Comey firing dominates Washington MORE (R-N.H.), Jeff FlakeJeff FlakeSenate should seek to retain its 'blue slip' tradition for judicial nominees Progressives target Heller and Flake on Senate GOP bill The Hill's Whip List: Senate ObamaCare repeal bill 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.