Hillary Clinton campaigned on Thursday in Iowa as early voting began in the pivotal swing state, seeking to pry it away from Republican Donald Trump and spur turnout that could ultimately decide the presidency.

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.

"We are starting to vote in Iowa today," Clinton told a 2,000-strong rally in Des Moines.

"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.