"You just keep pushing yourselves to succeed in ways that just mystify and leave us all in awe," she said.
After arriving in Tucson, the first lady hugged and spoke at the airport with Tucson children and teens who volunteer at a local urban farm before leaving for the fundraiser.
Mrs. Obama said that the election will be about the country asking, "Who are we?"
"The choice we make will determine nothing less than who we are as a country, but more importantly, who we want to be," she said. "Will we be a country where opportunity is limited to just a few at the top? Or will we be a place where if you work hard, you can get ahead no matter who you are or how you started out."
"All the indicators are that Arizona's in play," said Jim Haynes, president of the nonpartisan, Phoenix-based Behavior Research Center, which conducts election polls.
"The voters are obviously restless and wrestling with what they're going to do in November," Haynes said. "There's still a lot of question marks in their minds. And as things unfold positively and negatively on behalf of each one of them, I think people are going to bounce back and forth."