FAIRFAX, Va.—Hillary Clinton’s once-commanding lead among young voters has nearly collapsed, several polls show, a factor making the presidential race much closer in recent weeks and prompting the Clinton campaign to move quickly to keep a core Democratic constituency in the fold.

In its most visible response, the campaign has begun sending the party’s most popular stars to college campuses to urge students not to sit out the election or back third-party candidates, who are drawing support from young voters.

“Elections aren’t just about who votes, but who doesn’t vote, and that is especially true for young people like all of you,’’ first lady Michelle Obama said Friday during a campaign event at a university in Virginia, a battleground state where polls show the race tightening.

Vermont Sen. Bernie Sanders, a sensation among younger voters during the Democratic primaries, will campaign for Mrs. Clinton this weekend, and Massachusetts Sen. Elizabeth Warren, a liberal icon, will attend events this weekend at two Ohio universities.

Mr. Sanders previewed his message on Friday, saying he will urge young voters to look past the candidates’ personalities and instead consider Mrs. Clinton’s proposals for debt-free college and for raising taxes on the wealthy to fund government programs.