While Ms. Stein condemns both “zombie political parties” as tools of Wall Street and war profiteers, her campaign has focused largely on hammering Ms. Harris, blaming the White House she serves for relentless violence in Gaza and Lebanon.

And Democrats, as never before, are focused on Ms. Stein.

The party has prepared a negative ad blitz for the election’s final weeks, its first-such effort ever directed at a third-party candidate. Fearful that Ms. Stein might divert critical votes in places like Michigan, Democrats are also pressing their case on billboards plastered recently across swing states:

“Jill Stein Helped Trump Once. Don’t Let Her Do It Again.”

She dismisses the “spoiler mythology” that has come to define her mainstream identity, noting — accurately enough — that some of her supporters would never back Ms. Harris anyway.

She says that Democrats would do well to look inward, disputing that she bears any responsibility for Mr. Trump’s fortunes, then or now.

“Those conversations never go anywhere,” Ms. Stein, 74, said in a wide-ranging interview.