New York City Mayor Michael Bloomberg sent a letter Wednesday asking the state’s affluent Democratic campaign donors to cut financial support for senators opposed to the latest gun control bill brought to the Senate floor.

“I am writing to ask you: the next time these four Senators want you to support them with donations to their campaigns, tell them you cannot,” Bloomberg wrote, according to a Washington Post copy of the letter.

“Until they show that they will stand up for the American people and not the gun lobby, tell them you cannot support their candidacy,” Bloomberg continued. “These ‘no’ votes were a slap in the face to Americans everywhere.”

Sens. Mark Begich (D-Ala.), Mark Pryor (D-Ark.), Max Baucus (D-Mont.) and Heidi Heitkamp (D-N.D.) attracted Bloomberg’s politically-influential ire after siding with the majority of Senate Republicans that voted to successfully filibuster a bill that would have expanded background checks on gun purchases.

Senate Majority Leader Harry Reid (D-Nev.) became the fifth Democrat to vote against the bill, which he was in favor of — but only to recall it to the Senate floor later. The bill, which came in response to the Sandy Hook school shooting last December, was considered nationally popular according to polls Bloomberg cited in the letter.

“Polls consistently show that 90 percent of Americans — including 82 percent of gun owners and 74 percent of NRA members — support requiring background checks for all gun sales. But instead of standing up for what’s right, and for a common-sense measure their constituents support, these Senators made a calculated vote designed to pander to the gun lobby in anticipation of their next election,” Bloomberg said.

Three of the four face a consistently tough re-election cycle in notoriously red states, while Sen. Baucus plans to retire at the end of his current term. Bloomberg sent the letter to more than 1,000 wealthy Democratic campaign donors, who collectively provided more money for Democratic senators and the Democratic Senatorial Campaign Committee in the last election cycle than donors in every other state.

Bloomberg co-chairs the anti-gun group Mayors Against Illegal Guns, which continues to spend hundreds of thousands on attack ads against Republican and Democratic senators that voted to filibuster. The same organization has also supported senators that voted in favor of it — including Sen. Pat Toomey (R-Penn.), who co-sponsored the bill.