Michael Bloomberg (left), Oct. 2, 2013, in New York City, and Cory Booker, Feb. 2, 2013, in New York City. (Jemal Countess, Taylor Hill/Getty)

New York City Mayor Michael Bloomberg has committed $1 million to boost Newark Mayor Cory Booker's faltering Senate campaign in New Jersey with a new ad campaign.

The expenditure comes at a time when the lead Booker had over a relatively unknown Tea Party-backed Republican Steve Lonegan is shrinking. The ad was first reported Sunday by the New York Times.

Booker's lead over Lonegan went from 25 percentage points in an August Quinnipiac poll to just 12 percentage points in a poll conducted in late September.

The ad praises Booker for his work to challenge the National Rifle Association on gun-related issues, and his efforts to work with Republican Gov. Chris Christie to reform Newark public schools. It is set to air in the New York City and Philadelphia television markets.

"Mayor Booker is willing to try innovative new solutions - and he doesn't worry about party lines or the party's traditional special interests," said Stu Loeser, a Bloomberg spokesman. "And Mayor Bloomberg thinks we need a lot more of that in Washington."

The ads are being paid for officially by Bloomberg's PAC, Independence USA. But the billionaire mayor is the PAC's only donor. You can watch the full ad here.

It had initially seemed that Booker, a rising star in the Democratic Party whose Twitter prowess had given him far greater reach than the average city mayor, would coast to victory. But the recent tightening in the polls, combined with some recent missteps, including the revelations that he had engaged in banter with a stripper on Twitter, have raised concerns that he is under-performing against a far-right candidate in a Democratic state.