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?…

Mrs. Clinton’s dogged pursuit of Republican votes has especially rankled progressives, and highlights the divisions within the Democratic Party, even as they see a victory more likely. They have grumbled at her eager promotion of endorsements from veterans of the George W. Bush and Reagan administrations, including that of John D. Negroponte, a top diplomat and intelligence official under Mr. Bush. They worry aloud that Henry Kissinger, of whom Mrs. Clinton has often spoken fondly, could be next.

“Secretary Clinton’s decision to aggressively court Mitt Romney’s base has her looking more and more like Mitt Romney every day,” said Benjamin T. Jealous, a former N.A.A.C.P. president who initially supported Mr. Sanders. “That’s not a good thing.”