Mr. Bloomberg was not overlooking smaller states, either. He had booked $52,000 in ads, for instance, in Fargo, N.D. and $59,000 in Biloxi, Miss.

Mr. Bloomberg has not officially announced he is running for president, though this week he filed a formal “statement of candidacy” with the Federal Election Commission. He had previously filed to be on the ballot in three states.

His expected entry has injected a new element of uncertainty into the race, highlighting its fluidity as well as the angst among many moderate Democrats that the field has moved too far left, and that former Vice President Joseph R. Biden Jr., despite leading in most national polls, is poorly positioned to win. It has also stirred a debate among Democrats about the role of wealth in the primary and accusations from progressive rivals that Mr. Bloomberg is trying to buy the presidency.

“Swell,” Steve Bullock, the governor of Montana who is running a shoestring campaign for president, wrote on Twitter Friday. “Another Billionaire who thinks the Democratic nomination is for sale.”

Campaigning in New Hampshire, Senator Amy Klobuchar of Minnesota joked that she would also be announcing a multimillion ad buy.

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“No, I’m not,” she said, with a laugh. More seriously, Ms. Klobuchar argued that Mr. Bloomberg’s wealth could turn off some primary voters, who are tired of big money in politics. “They are tired of all the money in our world that’s at the top and I don’t think they want that at the top of our country,” she said.

The ad buy is the latest in a series of steps that Mr. Bloomberg, who has flirted with past presidential runs but has always demurred, has taken before formally declaring his bid.