Bloomberg-backed group goes after Flake on gun control

In the 30-second spot, Caren Teves, whose son Alex was killed in the Aurora shooting, shares a letter she wrote to Flake asking him to support expanded background checks on gun buyers.

He wrote back and suggested he agreed with her on the need for background checks, but ultimately voted against the Toomey-Manchin bill that would have instated them, which failed in the Senate.

 "The issue isn't just background checks, it's keeping your promise, and Senator Flake didn't," she says.

The ad is running in the Phoenix and Tucson media markets through May 30.

It's the third ad from the group to go after Flake, who's considered a potential vote in favor of expanded gun control if supporters are able to bring the measure back up in the Senate.

He responded to the ad in a post on his Faceboook, asserting that his vote for an alternate gun-control proposal by Sens. Chuck Grassley (R-Iowa) and Ted Cruz (R-Texas) indicated he did support background checks.

"If you are anywhere close to a television set in Arizona in the coming days, you’ll likely see an ad about gun control financed by NYC Mayor Bloomberg," Flake wrote. "Contrary to the ad, I did vote to strengthen background checks."

Flake's popularity dropped after Teves came forward with Flake's note and charged him with political opportunism.

A survey from Democratic firm Public Policy Polling showed him to be the least popular senator in the nation in April. Following the release of the poll, Flake acknowledged that his numbers were probably hurt by his vote against the background checks bill.

"…Notwithstanding the polling firm's leftist bent, I would assume that my poll numbers have indeed taken a southerly turn since my vote against the Manchin-Toomey background check proposal. It was a popular amendment, and I voted against it," he wrote on his Facebook.

MAIG has also targeted Sen. Kelly Ayotte (R-N.H.) with advertisements on her vote, as polling showed her vote made New Hampshirites less likely to support her.

--This post was updated at 1:47 p.m. to reflect Flake's response.