"Democrats who voted against the gun bill are in the doghouse in a big and important way with the progressive left in this country," said Neil Sroka, spokesman for Democracy for America.
The gun vote alone might not be enough to elicit sustained backlash from the Left; some progressives quietly concede that although emotional, gun politics only emerged on the political scene in January following the shooting massacre at Newtown, Conn.
Democrats, who assiduously avoided gun politics for more than a decade before the December tragedy, are more likely to be engaged on issues they have worked on much longer, like immigration or entitlement reform. To boot, many liberals say that even if it had passed the Senate, House Republicans would likely have prevented the gun measure from ever becoming law.
Many progressive activists say Democratic incumbents will face a sterner backlash if they support cutting entitlement programs like Social Security and Medicare. That's potentially critical because Obama has already publicly supported a plan for switching to a so-called "chained CPI" for Social Security benefits that could become a centerpiece of a major deficit-reduction deal.
"A vote against our safety net will rate higher than [the gun-control vote]," said Markos Moulitsas, a liberal activist and commentator. "We consider protecting our social net to be a core Democratic issue. If you don't believe in doing that, I don't understand why you'd even be a Democrat. And given how popular Social Security and Medicare are, it's not as if it's politically courageous to support those programs."
According to Sroka, progressive groups have drawn a "line in the sand" on reducing entitlement benefits.
"Anyone up in 2014, and really any Democrat period, needs to know, if they do vote to cut Social Security, Medicare, or Medicaid benefits, it's going to be a clarion call to the entire progressive community," he said. "It will fire up the grassroots base of Democratic Party."
Even if they don't draw a primary challenge, left-leaning activists say the senators will hurt themselves by caving to conservative pressure because of the effect it will have on their liberal base. Those supporters are often the ones most inclined to donate money or volunteer for a campaign — instilling the kind of enthusiasm many campaigns count on.
"No matter what, people want to feel good about a person they're volunteering for or donating to," said Adam Green, cofounder of the Progressive Change Campaign Committee. "So even Democratic voters who hold their nose to vote for an out-of-touch Democratic incumbent might be less likely to volunteer or donate, which could make a huge difference in a close election."
Not all red-state Democrats have engendered the Left's wrath. Sens. Mary Landrieu of Louisiana and Kay Hagan of North Carolina, for example, each backed the gun-control measure. Begich, along with Hagan, also supports same-sex marriage. Perhaps the progressive Left's No. 1 Democratic target, Sen. Max Baucus of Montana, this week announced his retirement.
And a "grand bargain" deficit deal — one that includes cuts to Social Security — remains a long shot, meaning these Democrats might never have to risk a potentially damaging vote.