What’s the difference between Mike Bloomberg, Joe Biden, Pete Buttigieg and Amy Klobuchar? That’s the question. On it rests all the prospects for the former New York mayor’s bid for the presidency.

This week marked Mr. Bloomberg’s debut on the national stage. Up to now, the Bloomberg campaign has been entirely prepackaged—more than $400 million worth of ads, canned statements and stump speeches in which Mr. Bloomberg refuses to answer questions.

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