The businessman-turned-populist stumped in Iowa a day earlier, appealing to white, blue-collar workers who have helped push him into the lead in the Hawkeye State, where the latest polls put him up nearly five points.

Iowa has long been an essential staging post on the path to the White House. Barack Obama's primary win in Iowa in 2008 propelled him to a thumping presidential victory that year and in 2012 that all but secured his re-election.

But Clinton has seen a decade-long Democratic advantage reversed, and it now appears like one of the toughest of seven swing states for the former secretary of state to win.

Her trip here coincides with the launch Thursday of non-postal early voting, which both campaigns see as a potentially critical opportunity for voters to cast ballots in person over the next several weeks.

"We are starting to vote in Iowa today," Clinton told a 2,000-strong rally in Des Moines that at times struggled to drown out a small but voluble gaggle of Trump protestors.

"We have 40 days to win an election that is going to affect the next 40 years of our country. "You, every one of you, can make the difference in this election," she said hitting Trump for bilking contracts in a message the campaign hopes will resonate with Iowans famed for their messianic fairness.

Locking down as many as half of all votes now could help the campaign tailor time and resources as the election enters the final stretch. But more vital for Clinton will be to ensure that chunks of the electorate actually turn out to vote and reverse Trump's lead.

Trump is most likely to win if the coalition of young, African American and Latino voters who voted for Obama decide to stay at home on November 8.

Clinton's main tool to inoculate against that possibility is likely to be Trump himself. Her campaign has framed the election as an existential moment for the republic: a choice between Clinton's safe pair of hands and the latent authoritarianism of an erratic and duplicitous mogul.

Clinton hammered away at Trump's propensity to stretch the truth, as her camp released a video highlighting several moments during the pair's first debate when Clinton pointed to past Trump statements, only for him to insist he never made them despite clear evidence.