A group advocating stricter gun laws that is financed by New York City Mayor Michael Bloomberg (I) released a new TV ad in Florida slamming Sen. Marco Rubio (R). The spot hits Rubio over his opposition to a Democratic proposal to expand background checks and goes after his willingness to filibuster gun control legislation.

"Marco Rubio, a new senator, but already looking past Florida, thinking of running for president," says the narrator of the spot the nonprofit group Mayors Against Illegals Guns is airing. "And that has Rubio running to the right, opposing a bill on comprehensive background checks."

The narrator adds: "Even worse, Rubio has said he would filibuster the vote in Washington."

Rubio is part of a group of more than a dozen Republican senators who've threatened to filibuster Democrats' gun control measures, including a proposal to expand background checks.

It's unclear, however, whether Rubio will support an alternative proposal to expand background checks that has bipartisan support. Sens. Joe Manchin (D-W.Va.), Pat Toomey (R-Pa.), unveiled the new proposal on Wednesday morning.

A strategist with Mayors Against Illegal Guns said the group is spending six-figures to air the Rubio spot statewide.

Bloomberg co-founded Mayors Against Illegal Guns, and is the group's biggest source of funding. The New York City mayor is one of the nation's most outspoken gun control activists. His group has spent millions on a national ad campaign pressuring both Democratic and Republican lawmakers to vote for stricter gun laws.