Senate Majority Leader Harry Reid’s aides met recently with staffers of New York Mayor Michael Bloomberg to warn them: Targeting vulnerable Democrats like Arkansas’s Mark Pryor on gun control could backfire on the party, several sources told POLITICO.

It didn’t work.

Ads from the Bloomberg-funded Mayors Against Illegal Guns are going up soon in Alaska, Arkansas and North Dakota — three states with Democratic senators who broke with the White House on last month’s background checks vote.

The group is also moving as many as 60 field organizers into about a dozen states where senators — Democrats and Republicans — voted against bill, with the goal of building infrastructure and countering gun rights groups like the National Rifle Association.

It’s all got Democrats nervous about keeping their hold on the Senate, if they are under attack from not only Republicans but pro-gun control forces as well.