Here's a clear sign that Democratic nominee Hillary Clinton is increasingly worried about her eroding position as front runner in the presidential race: She is making plans to aggressively court voters supporting the two most prominent third-party candidates.

Team Clinton is concerned that Republican nominee Donald Trump has been making real gains, Democratic sources say. He is closing in on Clinton nationally, according to recent polls, although she still holds a small lead in most surveys. Trump is also gaining or has pulled ahead in key battleground states such as Florida, Iowa, Michigan and Ohio. Clinton and her allies are particularly concerned that some millennials--generally defined as young voters born between 1981 and 1996--are moving toward Libertarian Party nominee Gary Johnson and Green Party nominee Jill Stein.

Clinton strategists have said for many months that they expected the election to be close. And that's what has happened. Clinton's strong lead after the Democratic National Convention in August has dissipated because of recent setbacks. During the past week alone, she nearly collapsed at a public ceremony because she had pneumonia; she suffered a backlash when she derided some of Trump's supporters in harsh terms, such as calling them racists and sexists, and she has had continuing problems proving her trustworthiness.

Priorities USA Action, a political action committee supporting Clinton, will undertake a multimillion-dollar digital campaign to solicit backing for her among those who are considering the third-party candidates, according to the New York Times. Clinton will also make special pitches next week in Pennsylvania, Ohio and Florida for millennial voters and liberals who now support or lean toward Johnson or Stein.

It appears that Clinton and her allies won't go negative against Johnson or Stein. Instead they are expected to argue that a vote for either of them is really a vote for Trump, who generates hostile reactions from many millennials and liberals.

Among the surrogates who will attempt to court millennials and liberals on Clinton's behalf are Sens. Bernie Sanders of Vermont, who lost the Democratic presidential nomination to her this year but still has a strong following in those groups; Sen. Elizabeth Warren of Massachusetts, a Democrat who also is popular with those voters, and former Vice President Al Gore, the unsuccessful Democratic presidential nominee in 2000 and a strong advocate of fighting global warming, a popular position among millennials. Gore plans to remind voters that Ralph Nader, the Green Party candidate in 2000, drew enough votes from Gore in Florida to cost the Democratic nominee the state and the election, the Times reports.