To say that Bernie Sanders and Michael Bloomberg are different is one of those statements that is at once both entirely true, yet woefully insufficient.

For all the differences that separate the Vermont senator and the former New York mayor, though, the most profound one is very simple: They offer divergent theories of what the 2020 presidential election is all about. Mr. Sanders believes Democratic voters are ready to overthrow the system. Mr. Bloomberg thinks they merely want to overthrow President Trump.

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