Jerrold Nadler

Michael R. Bloomberg, New York City’s billionaire former mayor, is preparing to spend millions of dollars on a super PAC boosting one of his former aides in the race for a coveted Manhattan House seat, according to people familiar with his plans.

The intervention is designed to help the former aide, Assemblyman Micah Lasher, distinguish himself from a crowded and star-studded field of Democrats competing to succeed Representative Jerrold Nadler, who is retiring.

Mr. Bloomberg publicly endorsed Mr. Lasher Thursday afternoon. But the people familiar with his plans, who were not authorized to discuss them, said that the former mayor also planned to follow up with a spending blitz to promote Mr. Lasher via mail, TV and digital advertising.

The planned ad campaign, which has not been previously reported, could begin in a matter of days. One of the people said the spending could total around $5 million. Other details were not immediately available.

The primary contest was already shaping up to be one of the most closely watched and expensive in the United States. The district is safely Democratic, but is among the wealthiest and most educated in the country, home to Fortune 500 companies, Broadway, media empires and a large network of donors.

Ambitious Democrats have crowded in ahead of the June primary. They include Jack Schlossberg, the grandson of President John F. Kennedy; George Conway, a former supporter of President Trump turned high-profile critic; Assemblyman Alex Bores; and Nina Schwalbe, a public health expert.

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