The crowd chanted “I love you” as first lady Michelle Obama walked onto the stage in Phoenix, an appearance she used to attack Hillary Clinton's rival for the US presidency, Republican candidate Donald Trump.
Clinton is hoping to win in Arizona at the November 8 election, something her party has done only once since 1952.
She enjoys an unexpected five per cent lead over Trump in the state's polls, but 21 per cent of voters remain undecided.
Mrs Obama criticised Trump for comments he made during Thursday's presidential debate that he would keep voters "in suspense" about whether he would accept the election results.
“You do not keep American democracy 'in suspense', too many have fought and died to protect this democracy,” Mrs Obama said.
“While she [Clinton] has detailed policies. He has Tweets, you decide.
“If Hillary Clinton doesn’t win this election, it’ll be on us.”
The Democrats have been campaigning heavily in the country’s sixth largest state, with Clinton’s former rival for the party's presidential nomination, Bernie Sanders, addressing a large crowd on Tuesday, and Clinton's daughter Chelsea speaking on Thursday.
It’s believed Trump’s tough talk on immigration has scared off some Hispanic voters, who make up 30 per cent of the state’s population.
Campaigning for her mother, Chelsea Clinton talks to a crowd at Arizona State University (AAP) Source: AAP
Arizona congressman Ruben Gallego is confident of a Democratic win, claiming: “We are now officially a swing state.”
Arizona voter Carolina Coronel said: "What scares you about Donald Trump? ...I'm a Mexican and when he said drug dealers – that’s not me – that scared me.”
