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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 Ann AyotteDems plan to make gun control an issue in Nevada Stale, misguided, divisive: minimum wage can't win elections Trump voter fraud commission sets first meeting outside DC MORE (R-N.H.), Jeff FlakeJeffrey (Jeff) Lane FlakeAuthorizing military force is necessary, but insufficient Republicans jockey for position on immigration McCain, Flake warn against 'politically-motivated penalties' for Canadian defense firm 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.