Michelle Obama, Bernie Sanders hit the campaign trail for Hillary Clinton

Bernie Sanders and Michelle Obama are hitting the campaign trail,urging young voters to vote for Hillary Clinton and keep Donald Trump out of the White House.

The president's wifeon Friday warned young voters against being "tired or turned off" in the 2016 election. She urged them to rally behindClinton, the Democratic candidate, "particularly given the alternative."

Obama is emerging as one of Clinton's most effective advocates, especially with voters who backed her husband,but are less enthusiastic about his potential Democratic successor.

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Friday's rally in Virginia was Obama's first solo campaign event for Clinton and comes nearly two months after her star turn at the Democratic convention.

Speaking to mostly students at George Mason University, she repeatedly jabbed Republican rivalTrump without mentioning him by name, declaring that being president "isn't anything like reality TV."

Young voters not thrilled about Clinton

Polls show a tightening race, and there is fear in the Clinton campaign thatmillennialslack enthusiasm forthe Democratic candidate. It may have been illustrated unintentionally on Friday by university students who greeted MichelleObamaat a campaign rally with chants of"Four more years!"

That enthusiasm gap threatens Clinton's chances in key swing states. Sanders is stumping in two of them: Pennsylvania and Ohio. The Vermontsenator is urging supporters to back his former primary rival instead of staying home or supporting a third-party candidate.

"I think that before you cast a protest vote because either Clinton or Trump will become president think hard about it," Sanders told MSNBC'sMorning Joeshow on Friday.

"This is the presidency of the United States. And I would say to those people out there thinking of the protest vote, 'Think about what the country looks like, and whether you're comfortable with four years of a Trump presidency.'"

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His involvement in the campaign comes two months after he appeared with Clinton at a rally to offer his endorsement and one month after groups of his supporters heckled Clinton at the Democratic convention.

But Sanders said his supporters should realize that on issues that matter, Clinton's platform is far closer to his. He compared Trump's tax policies that would transfer wealth toward the rich with her plan to eliminate tuition at public college: "You know what? That [tuition plan is]pretty good. That is pretty damn good."

Strategic voting

A New York Times report says former Democratic nominee Al Gorecould also enter the campaign, to remind a younger generation of the consequences a few Green Party votes may have had in 2000: eight years of George W. Bush and a disastrous war in Iraq.

Gore lost the presidency by just 537 votes in Florida. Meanwhile, the Green Party's Ralph Nader won 97,488 votes in that state. Many Democrats blame Nader for that defeat, which Green supporters resent.

Many Sanders supporters have said in interviews that they could vote Green. At one of the final rallies of the Sanders campaign, in California, Kate Tanaka said she'd always voted Green and would do so again if Sanders wasn't on the ballot.

Yet some also said they're thinking strategically.

Tanaka said some voters like her might act differently if they live in a crucial state. That's not the case in California, where Democrats tend to win easily. "It's not a real gun-to-the-head situation [here in California]," Tanaka said.

Mary Andriotakis offered a similar view. At a rally outside the Democratic convention, the Sanders supporter says she could afford a protest vote because she lives in solidly Democratic Massachusetts.

If she lived in Ohio, she said,"things might be different. I'd have to make some hard choices."

Sanders had events scheduled in Ohio on Saturday, after a stop in Pennsylvania on Friday evening.

Clinton's aides, meanwhile,want MichelleObama in battleground states as much as possible between now and election day.

• Michelle Obama gives impassioned endorsement of Clinton

Friday's rally in northern Virginia, less than an hour drive from the White House, is the only event she's publicly committed to, though the Clinton campaign expects her to make additional appearances.

Jennifer Palmieri, Clinton's communications director and a former Obama adviser, called the president's wife "an advocate without peer."

"There is no other surrogate with the reach, credibility and respect she has," Palmieri said.