“We’ll be launching a multimillion-dollar digital campaign that talks about what’s at stake and how a vote for a third-party candidate is a vote for Donald Trump, who is against everything these voters stand for,” said Justin Barasky, a strategist for Priorities USA.

Mrs. Clinton may also get an assist from one Democrat who has been largely quiet about the race, but can testify to the importance of resisting the third-party temptation: former Vice President Al Gore. Her staff has had conversations with aides to Mr. Gore about bringing him onto the campaign trail to emphasize the importance of supporting Mrs. Clinton if they want to make progress on combating climate change.

“I can assure you from personal experience that every vote counts,” Mr. Gore wrote in an email to The New York Times on Thursday, after a new CBS/New York Times poll showed Mrs. Clinton and Mr. Trump virtually tied. “The stakes are high for so many Americans. So I will vote for Hillary Clinton and I strongly encourage others to vote for her as well.”

More immediately, the Clinton campaign on Saturday will dispatch two political figures who enjoy a passionate following among young liberals, Senators Bernie Sanders and Elizabeth Warren, to Ohio, where public polls show Mrs. Clinton has slipped into a statistical dead heat with Mr. Trump. And Mrs. Clinton will deliver a speech aimed at millennial voters on Monday in Philadelphia before campaigning and giving an economic address in Florida later in the week.

Democrats say that if the race is close in its final stretch, some of the voters who do not want to see Mr. Trump elected may shift on their own accord to Mrs. Clinton to prevent a Trump presidency. But after spending much of the summer hammering Mr. Trump, through both ads and stump speeches, it appears Mrs. Clinton has convinced many voters that Mr. Trump is not qualified to be president but has failed to win them over to her own candidacy.