Gun control supporters are continuing to target Sen. Jeff Flake, with a group led by New York Mayor Michael Bloomberg (I) launching a new ad buy against the Arizona Republican.

The Mayors Against Illegal Guns ad features the mother of a victim of the movie theater shootings in Aurora, Colo., Caren Teaves. She criticizes Flake for voting against the Senate's background checks legislation after telling her in a personal letter than he supported expanding them.

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

A spokeswoman for MAIG said the ad buy is six figures but wouldn't specify beyond that.

Flake has admitted that he has noticed the criticisms of his vote.

"Nothing like waking up to a poll saying you're the nation's least popular senator," he wrote on Facebook shortly after the background checks vote. "Given the public's dim view of Congress in general, that probably puts me somewhere just below pond scum."

Gun control advocates have focused their efforts on recent weeks on Flake and another GOP senator, Kelly Ayotte (N.H.), whose numbers have also dipped since the vote. Their hope is that Ayotte and Flake may support a renewed effort on background checks.

Flake, for what it's worth, was just elected to his first term and won't face reelection until 2018.