Because it worked so well when he did the same for Colorado Democrats facing recall elections.

Via NY Times:

Cory A. Booker is an undisputed star of a new generation of African-American leaders, electrifying liberal audiences with his oratory and charming the social media set with his digital savvy.

But the Senate campaign Mr. Booker, a Democrat, is running in New Jersey — at times sputtering, unfocused and entangled in seemingly frivolous skirmishes over Twitter messages involving a stripper — has unnerved his supporters, who thought that a robust and unblemished victory over his Republican opponent, Steve Lonegan, would catapult him onto the national stage.

As his allies move to shore up what was supposed to be a painless path to Congress, the biggest and wealthiest of them, Mayor Michael R. Bloomberg, will start spending more than $1 million on Monday to broadcast television commercials on Mr. Booker’s behalf, a vast sum to pour into a single candidacy.

With the special Senate election next week, the late campaign push is trying to exploit unhappiness over the government shutdown to promote Mr. Booker, Newark’s mayor, as a problem solver who eschews the kind of hard-line partisanship crippling the Capitol. “He is a solutions guy rather than an ideologue,” Mr. Bloomberg said in an interview.