Michael R. Bloomberg still has not declared whether he is running for president in 2020. He is about to become the single biggest spender in the presidential race anyway.

Ahead of a potential campaign announcement, Mr. Bloomberg, the billionaire businessman and former mayor of New York City, is beginning a $100 million digital campaign designed to attack and define President Trump in the top battleground states seen as likely to decide the 2020 election. The ads will go online on Friday in four states — Arizona, Michigan, Pennsylvania and Wisconsin — and run through the end of the primary season, even if Mr. Bloomberg is not in the race.

In a twist, Mr. Bloomberg himself will not be featured in the ads beyond legally required disclaimers, even as he actively lays the groundwork for a campaign, his advisers said. The $100 million ad buy will be in addition to whatever Mr. Bloomberg may spend on efforts to promote himself to become the Democratic nominee.

“We will have other ads that feature Mike Bloomberg,” said Howard Wolfson, a top political adviser to Mr. Bloomberg. “You will be seeing a lot of Mike Bloomberg.”