Seventeen months before Election Day, Arkansas Democrat Sen. Mark Pryor is already outraged at his opposition. “Disgusting,” he said in a statement released Friday, when a group funded by New York Mayor Michael Bloomberg bought $350,000 in television time for an ad aimed at denying Pryor votes among the state’s heavily Democratic African American community.

Pryor isn’t the only one. Senate Democrats up and down the caucus, from West Virginia’s Joe Manchin to Majority leader Harry Reid of Nevada, have been trying to warn Bloomberg off his strategy of running ads that attack vulnerable Democrats over gun control votes. And it’s not because they disagree with what Bloomberg is after—a new law to require background checks for guns bought online or at gun shows. Rather, they think keeping the Senate in Democratic hands in 2014 is more important than any single Senator’s vote on guns.

As it now stands, Democrats have a six-vote cushion in the Senate, the result largely of Republican division and poor candidate selection. That cushion could easily be swept away in 2014. According to early handicapping, there are 13 Democratic seats at risk, and just two Republican seats. And the Democratic seats in play, in states like South Dakota, West Virginia and Arkansas, are in far greater danger of slipping away than the Republican seats in Georgia and Kentucky.

(MORE: Behind Bloomberg’s Gun-Buyer Background-Check Ad Blitz)

Bloomberg’s consultants, operating through a group called Mayors Against Illegal Guns, know this, but they don’t care. There aims are decidedly non-partisan, they say. Their goal is to establish a consequence for vulnerable members who vote against gun control, just as the National Rifle Association do to those who vote for stricter gun control measures. Mayors Against Illegal Guns wants a scalp, and Pryor, a Democrat up for reelection in a state that President Obama lost by 24 points, can’t afford to lose his base. “It is hard for me to imagine a combination of constituencies that would get Mark Pryor over the finish line if he doesn’t perform exceptionally well in the African American community,” said Mark Glaze, the public face of the Bloomberg group in an interview with the Associated Press.

The disagreement poses a dilemma for supporters of gun control for which there is no clear answer: Is it better to teach wavering Democrats that there is a cost to voting against gun control, even if it jeopardizes Democratic control of the Senate, which is needed to enact gun control? Or is it better to maintain Democratic Senate control even if it means that some red-state Democrats are more likely to vote against gun control to protect their right flank?

President Obama took a position on this question in April, when he declined to lean heavily, or publically call out, the four Democratic senators who voted to block the expansion of background checks. While pundits attacked him as a weak leader, unable to sit down Alaska Sen. Mark Begich for a stiff drink, White House aides spoke privately about the importance of maintaining a Democratic Senate majority in 2014.

(MORE: An Angry Obama Lashes Out After Gun Control Defeat)

Since then, however, Obama’s own political organization, Organizing for Action (OFA), has promised to bring pressure on the four defecting Democratic senators—Pryor, Begich, North Dakota’s Heidi Heitkamp and Montana’s Max Baucus. Jon Carson, the head of OFA, has said his volunteers are less concerned about hurting the reelection chances of Democrats than getting a background check bill through Congress. In recent weeks, the group has held phone banks in Alaska, Arkansas and North Dakota to inform residents of their Democratic senators’ votes against more gun control. OFA volunteers have also protested outside Baucus’s office in Bozeman, even though Baucus has announced that he will not run for reelection after his term expires.

Pryor, meanwhile, has been trying to make the best of his situation, by joining in the evergreen strategy of attacking Bloomberg personally as a elitist carpetbagger. “I’ve gotten a lot of questions about NYC Mayor gun ad,” Pryor wrote in a tweet last month. “My response? I don’t take gun advice from the Mayor of NYC. I listen to Arkansans.”

After tweets like that, it will be difficult for Pryor to reverse himself if background check expansion comes up for another vote this year, without looking like he is caving to his New York foe—another consequence of the New York mayor’s take-no-prisoners strategy.

MORE: Gun Bill’s Prospects Dim as Senate Struggles with Background Checks