New York City Mayor Michael Bloomberg's gun control nonprofit is ramping up its campaign against Sen. Kelly Ayotte (R-N.H.), releasing a new ad accusing her of misleading constituents about her stance on gun background checks.

Bloomberg's group, Mayors Against Illegal Guns, is dropping $400,000 to run the ad, called "False Alibi", in the Manchester, N.H., and Boston media markets. The ad comes one month after MAIG spent $650,000 on ads slamming Ayotte's "nay" vote on the Senate's bipartisan background check bill.

In the 30-second spot, Scott Knight — a police chief in Chaska, Minn., and a prominent gun-control activist — claims that Ayotte falsely presented her support of a separate gun measure as proof she supported expanded background checks. In fact, that bill would have weakened those restrictions.

"Senator Ayotte uses her vote for this law as an alibi to claim she is tough on crime," he says in the ad. "Don't be fooled."

The ad is the latest in an escalating tug-of-war between Bloomberg and the National Rifle Association, which has launched its own ads — albeit at a much smaller price tag — to defend Ayotte.