WASHINGTON—The rift between Elizabeth Warren and Bernie Sanders has exposed a long-simmering frustration among Democratic women: A sense that female candidates have been sidelined because voters think a man is more likely to defeat President Trump.

On the heels of Hillary Clinton’s loss to Mr. Trump more than three years ago, the 2020 Democratic field has featured the largest number of women seeking the presidency. But that list of female contenders has withered. Of the four candidates bunched together at the top of polls…

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