Mayor Michael R. Bloomberg, in a sharp escalation in the battle over gun control, is seeking to punish Democratic senators by taking away the one thing they most need from New Yorkers: money.

On Wednesday, Mr. Bloomberg will send a personal letter to hundreds of the biggest Democratic donors in New York urging them to cut off contributions to the four Democratic senators who helped block a bill in April that would have strengthened background checks on gun purchasers.

The move could inflame tensions that have simmered for weeks between Mr. Bloomberg, who blames the four Democrats for the defeat of the bill, and Democratic Senate leaders, who have privately told City Hall that the attacks can serve only to empower a Republican majority openly hostile to Mr. Bloomberg’s priorities.

By appealing to the Democrats’ financial base, Mr. Bloomberg is exploiting his relationships and prestige among wealthy New Yorkers to disrupt the flow of campaign money to key Democrats whose re-election next year will help determine whether the party retains control of the Senate. No state is more essential to the party’s fund-raising: Sitting Democratic senators and the Democratic Senatorial Campaign Committee raised $30.4 million from New York donors in 2012, according to the Center for Responsive Politics, more than in any other state.