The significant expenditure comes as multiple polls show the race to fill deceased Sen. Frank Lautenberg's (D-N.J.) seat to be closer than expected. 

Republican Steve Lonegan remains the heavy underdog, but Booker's initial lead, in the high teens or low twenties, appears to have shrunk to low double-digits in recent weeks.

According to The New York Times, which first reported news of the ad, it will characterize Booker as a "senator to get things done" in a "divided Washington." The ads will run for 10 days starting Monday.

“He is a solutions guy rather than an ideologue,” Bloomberg told the Times.

Though the fundamentals of New Jersey — a blue state that President Obama won with nearly 60 percent of the vote last year — favor Booker, the odd timing of the election makes turnout difficult to predict, and a win impossible to guarantee.

Bloomberg's expenditure is an indication, however, that Democrats are willing to do whatever it takes in the final week to get him across the finish line.