TPM reached out to spokespersons for each of the five senators, whose positions on background checks are unclear, on Monday.
"Senator Donnelly is open to a bipartisan compromise on background checks, one that helps keep guns out of the hands of criminals and the mentally ill while protecting our Second Amendment rights," said Donnelly spokeswoman Elizabeth Shappell.
Heitkamp spokeswoman Whitney Phillips said: "Sen. Heitkamp is reviewing the current proposal on the table as well as other alternative proposals currently being worked on." Phillips said Heitkamp is in North Dakota this week for recess, where she will be "hearing from constituents about this issue and the many others facing our state and country."
Donnelly and Heitkamp were first elected last November in conservative states. Pryor, Landrieu and Hagan are up for re-election in 2014 and are prime targets for defeat by Republicans.
"As the debate moves to the Senate floor, Senator Hagan will be looking for a commonsense place to start this discussion, which could be background checks, so long as it doesn't infringe on the rights of responsible gun owners," Hagan spokesman Christopher Moyer told TPM. "Senator Hagan always votes for North Carolina first, and she will continue listening to the voices of North Carolinians."
In the afternoon, Pryor responded with a sharp retort to Bloomberg. "I don't take gun advice from the Mayor of New York City," he said. "I listen to Arkansans." In a follow-up statement to the Washington Post, his spokesman didn't shut the door on background checks.
Landrieu's spokesman Matthew Lehner emphasized her "strong record of support for the Second Amendment" and the need to "protect the constitutional rights of law-abiding gun owners." He added, "She looks forward to working with law enforcement officials, who want to see additional precautions taken to keep weapons and handguns out of the hands of criminals."
The ad buy is an effort by the New York City mayor and outspoken gun control advocate to pressure fifteen senators from both parties "where they can most influence the upcoming Senate vote." Bloomberg's group Mayors Against Illegal Guns announced Saturday that it will air two ads in some dozen states featuring a gun owner who describes background checks as compatible with defending the Second Amendment and his family.
The Republicans being targeted are Sens. Jeff Flake (AZ), Saxby Chambliss (GA), Johnny Isakson (GA), Dan Coats (IN), Chuck Grassley (IA), Susan Collins (ME), Kelly Ayotte (NH), Dean Heller (NV), Rob Portman (OH) and Pat Toomey (PA).
The senators' skittishness serves as a reminder that finding 60 senators to break an anticipated Republican filibuster on background checks will be a struggle. That's even though polls find a whopping nine in 10 Americans support the idea -- including an overwhelming majority of gun owners. It's a reminder that despite public relations missteps by the National Rifle Association in the wake of the Newtown, Conn., elementary school shooting last December, the gun industry lobby remains a force to be reckoned with.
Senate Majority Leader Harry Reid (D-NV) on Thursday moved to advance gun control legislation and said he'll push to include background checks in the final version.
"I hope negotiations will continue over the upcoming break to reach a bipartisan compromise on background checks, and I am hopeful that they will succeed," Reid said. "If a compromise is reached, I am open to including it in the base bill. But I want to be clear: in order to be effective, any bill that passes the Senate must include background checks."