The Democratic primary race has divided voters into two clear coalitions, one backing Sen. Bernie Sanders’s call for revolutionary change and one drawn to the quieter tone and more incremental policies proposed by former Vice President Joe Biden.

Super Tuesday supplied one surprising reason that Mr. Biden’s coalition, so far, is the larger one: President Trump, a force from outside the Democratic Party, is helping to change the Democratic electorate, making it more friendly to Mr. Biden’s style of politics.

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