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 AyotteAudit finds US Defense Department wasted hundreds of millions of taxpayer dollars US sends A-10 squadron to Afghanistan for first time in three years No, the US did not spend million on a gas station in Afghanistan MORE (R-N.H.), Jeff FlakeJeffrey (Jeff) Lane FlakeOvernight Health Care: House passes .3T omnibus | Bill boosts funds for NIH, opioid treatment | Senators spar over ObamaCare fix | 'Right to Try' bill heads to the Senate The Hill's 12:30 Report Booker admits defeat in Capitol snowball fight with Flake MORE (R-Ariz.) and Mark PryorMark Lunsford 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.