PHOENIX — First Lady Michelle Obama said Thursday that Donald Trump's debate-night refusal to say whether he'd accept the results of the election was "threatening the very idea of America itself," encouraging a packed arena to turn out to vote despite Trump's claims of a rigged election.

Trump's strategy, Obama warned, is "to make this election so dirty and so ugly that we just turn off the TV and say, 'We just don't want any part of it.'"

"So when you hear folks talking about a global conspiracy and saying this election is rigged, understand that they are trying to get you to stay home. They are trying to convince you that your vote doesn't matter, that the outcome has already been decided and you shouldn't even bother to make your voice heard," Obama continued.

"They are trying to take away your hope."

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Trump's claims of a rigged election — culminating in his declaration during Wednesday night's debate that he would "look at it at the time" when asked whether he'd accept the results of the election — have fueled an anti-election fervor among his supporters, many of whom warn at his rallies of an "uprising" if the GOP nominee loses.

They've also sparked fears among election experts that his rhetoric could challenge the peaceful transfer of power, a bedrock of American democracy since the United States was founded. Obama mentioned this in her speech, telling the crowd that Americans are "fortunate to live in a country where ... the voters decide who wins or loses, period, end of story."

"And when a presidential candidate threatens to ignore our voices and reject the outcome of this election, he is threatening the very idea of America itself," she said.

Drawing huge cheers from a crowd that the Clinton campaign estimated at 7,000 people, Obama declared: "We cannot stand for that. You do not keep American democracy in suspense."

It was a rousing speech, the likes of which the first lady has become known for as she's been an increasingly active presence on the campaign trail for Clinton. Last week, Obama made headlines for an emotional speech in which she rebuked Trump for his lewd comments about groping women in 2005.

Thursday in Arizona, she rebutted Trump's charges of a rigged election while reviving her husband's 2008 campaign theme of "hope," declaring Clinton the only candidate to offer it. Trump's vision, Obama said, is one "completely and utterly lacking in hope.

In contrast, "Hillary knows that our country is powerful and vibrant and strong, big enough to have a place for all of us and that each of us is a precious part of the great American story."

Related: Michelle Obama Takes on Trump: 'Enough Is Enough'

While Arizona is a traditionally red state, Trump's struggles have fueled Democratic hopes for a win there, and the first lady's visit was an indication of how seriously Democrats are taking the state.

During her speech Thursday, she emphasized the stakes by noting her husband lost Arizona in 2012 by just 208,000 votes — about 63 votes per precinct — and warned of the consequences if voters deliver the election to Trump by staying home.

"Think about how you'll feel waking up on November the 9th if that happens ... if you didn't do everything possible to get this done," she said.