With red state Democratic Senators remaining skittish about embracing Obama’s gun proposals, at least two top Democratic donors are stepping forward and vowing to withhold any and all future financial contributions from any Democrats who don’t support the centerpiece of Obama’s plan: Expanded background checks.

Kenneth Lerer, a New York businessman who is chairman of Buzzfeed.com, and David Bohnett, a technology entrepreneur and philanthopist based in Los Angeles, are both major financial supporters of Democratic candidates, having each given scores of large contributions over the years. They are both key players in the political fundraising world and wield influence among other donors and fundraisers.

Neither will give another dime to any Senate Democrat who does not support expanded background checks, I’m told — and both will suggest to other donors that they do the same. The move underscores the rising importance of gun control as an issue in Democratic politics — and the rising frustration in some Democratic circles with elected officials who continue to regard gun politics as a third rail, at a moment that presents a real opportunity to achieve serious reform, with a policy that enjoys near universal public support.

“At some point you have to draw a line in the sand — for me that time is now,” Lerer told me in an interview. “If candidates or officeholders can’t support something like comprehensive and enforceable background checks, then I wouldn’t think of giving them any money going forward.”

Lerer also said he would be intensifying his contributions to Democratic Senate candidates in the next few years — excluding any that don’t take a strong position on gun control. “We intend to get very active in Senatorial campaigns during the next cycle and the one after,” he said. Red state Dems up for reelection in 2014 include Mary Landrieu, Kay Hagan, Mark Begich, Mark Pryor, and Max Baucus.

Bohnett, too, will not give any more money to Dems who balk on guns. “If they don’t do the right thing on background checks, donors like David won’t be able to support them,” Michael Fleming, who advises Bohnett on his political and philanthropical giving, told me. “You look at what some senators from rather conservative states have done when it comes to supporting marriage equality — they ‘re willing to take a tough stand. We would expect nothing less on the gun issue.”

“We will certainly share our concerns with our fellow funders,” Fleming added.

Michael Bloomberg’s pro-gun-control group, Mayors Against Illegal Guns, has been informally reaching out to top Democratic contributors and encouraging them to evaluate candidates on the basis of their support for strong gun legislation, I’m told.

A third major fundraiser, Grace Tsau-Wu of Chicago, who was on Obama’s national finance committee in 2012, also tells me she won’t help any candidates who are weak on guns get connected with her network of donors. “It’s hard for me to personally fundraise or get involved in the campaigns of any Democrats who don’t support background checks and aren’t behind the president on the issue of guns,” she said.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.