Liberal Democrats and progressive activists have grown wary of the state of the 2016 presidential race, chafing at Hillary Clinton’s big-tent courtship of Republican leaders they have long opposed and fearing the consequences of shaping the contest as a referendum on Donald J. Trump.

While few have questioned the electoral strategy of bringing Republicans into the fold by casting Mr. Trump as a singular threat to democracy, both skeptics and some admirers of Mrs. Clinton have come to view her decisive advantage in the polls with mixed emotions.

She may win by a mandate-level margin, they say. But what, exactly, would the mandate be for?

In a matter of weeks, beginning with the party conventions, the policy-driven debates that animated the Democratic primary race have largely disappeared from the political foreground, giving way to discussions of Mr. Trump’s temperament, his inflammatory remarks and even his sanity.

“If she’s going to get anything done as president, she is going to have to have a mandate,” said Robert B. Reich, a secretary of labor in Bill Clinton’s administration who supported Bernie Sanders in the primary.