By Jeff Mason

FAIRFAX, Va. (Reuters) - Democratic presidential nominee Hillary Clinton's campaign deployed a popular surrogate on Friday to gin up enthusiasm amid young voters who could be critical to victory on Nov. 8: Michelle Obama.

The U.S. first lady and wife of President Barack Obama, who won the White House in 2008 and 2012 with high levels of support from young voters, told a crowd of students in Virginia that they could mean the difference between a Clinton win or a loss to Republican nominee Donald Trump.

"Let’s be clear, elections aren't just about who votes, but who doesn’t vote. And that is especially true for young people like all of you," she said, noting that voters under the age of 30 provided the margin of victory for Obama in the swing states of Ohio, Pennsylvania, Florida and Virginia in 2012.

"Without those votes, Barack would have lost those states and he definitely would have lost that election. Period, end of story," she said.

A New York Times/CBS poll this week showed Clinton with more support from people under 30 than Trump but still well below the levels Obama achieved.

Though she has not yet been a frequent presence on the 2016 campaign trail, Mrs. Obama has proven to be an especially powerful advocate for Clinton, her husband's one-time rival. Her speech in support of Clinton at the Democratic convention in July was praised as one of the best of that event.

“I’m inspired," she said, listing Clinton's positions in public service and rejecting arguments that Clinton was not an inspiring figure. The current first lady praised the former first lady as one of the most qualified people for the office of president in history.

"So we cannot afford to squander this opportunity, particularly given the alternative. Because here is what we know: that being president isn’t anything like reality TV," Mrs. Obama said, referring to Trump, a former reality television host.

A president could not just "pop off" when making life or death decisions about war and peace, she said without naming Trump.

Mrs. Obama noted that the country was in a time of transition, just like her family, which was about to move into a new house. She joked that they had to make sure the White House was cleaned up so they could get their security deposit back.

Then she referred to the transition in 2008 with a dig at Trump for questioning whether Barack Obama was born in Hawaii. On Friday he finally conceded that Obama was U.S. born.

"There were those who questioned, and continued to question for the past eight years, up through this very day, whether my husband was even born in this country," she said to boos.

"I think Barack has answered those questions with the example he’s set: by going high when they go low." 

(Reporting by Jeff Mason; Editing by Chris Reese)