Chicago, Illinois - Kamala Harris arrived in Chicago on Sunday ahead of her star turn at the Democratic National Convention, perhaps the most pivotal moment yet for her short but stunning election campaign against Donald Trump.

The vice president has reenergized the many in the party after an astonishing month that has seen her replace President Joe Biden at the top of the ticket and wipe out Republican rival Trump's lead in the polls.

On the way to Chicago, Harris stopped in the crucial battleground state of Pennsylvania – which Trump won in 2016 and Biden in 2020 – and embarked on a day-long bus tour in a bid to keep up the momentum.

Accompanied by her running mate Tim Walz and their respective spouses, they set off from Pittsburgh on a coach emblazoned with their names to a series of rust-belt towns to woo blue-collar voters.

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But Harris insisted that Trump was still the favorite to win and that she would campaign hard with just 79 days until the November 5 election.

"I very much consider us the underdog. We have a lot of work to do to earn the vote of the American people," Harris told reporters in Pennsylvania.

"That's why we're on this bus tour today."

Harris' rapid rise has unsettled 78-year-old former president Trump, who is resorting to his favored tactic of personal insults against America's first female, Black, and South Asian vice president.

A day earlier at a rally in Pennsylvania, Trump lashed out at Harris as a "lunatic" and bragged that he was "much better looking" than 59-year-old Harris.

Harris' headline convention speech on Thursday will now be a vital opportunity to sell her story to an electorate that is still getting used to the new name at the top of the Democratic ticket.

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She is expected to cast the election as a "fight for the future" against a second Trump term, while promising an optimistic vision for Americans struggling with high prices.

Harris is set to join Biden on stage when he addresses the convention on Monday – a speech that just a few weeks ago he expected to be giving as the Democratic candidate.

Biden is expected to focus on passing the torch and on what he terms the threat to democracy posed by Trump, as he seeks to cement his legacy by helping Harris to victory.

Harris' nomination has allowed many Democrats to hope again, just a few short weeks after a disastrous debate performance by Biden led many to think the election was already lost.

A Washington Post-ABC-Ipsos survey published Sunday showed Harris with a narrow lead over Trump among registered voters across the country, where one month ago it had Trump and Biden in a dead heat.

But the Biden-Harris administration's support for Israel's war on Gaza has sparked mass public outrage ahead of the four-day convention. Tens of thousands of people expected to descend on the city to hold daily protests, the biggest of which are set for Monday and Wednesday.

On Sunday, several hundred demonstrators gathered in downtown Chicago carrying a banner saying "Feminist and LGBTQ+ People for a Free Palestine."

"The march was going to be for reproductive rights, then the Gaza genocide exploded. That's why we're here," said 76-year-old David Finkel, from Michigan, who said he was also in Chicago for the infamous 1968 protests at the Democratic convention.

Illinois Governor J.B. Pritzker said the planned protests would be allowed as long as they remained peaceful.

"If there are troublemakers, they are going to get arrested and they are going to get convicted," he told CNN.