Pittsburgh, Pennsylvania - Kamala Harris took her surging US presidential campaign on a battleground bus tour Sunday, before heading to the Democratic National Convention for a star turn that will be shadowed by protests against Israel's assault on Gaza.

The vice president, riding a wave of enthusiasm after replacing President Joe Biden at the top of the ticket less than a month ago, dashed to the swing state of Pennsylvania to push her case against Republican rival Donald Trump.

Later in the day, the 59-year-old will head to Chicago for what is expected to be a rapturous reception from Democrats, who have dared to hope again after an astonishing turnaround that has seen Harris wipe out Trump's lead in the polls.

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

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Security has been ramped up for the Chicago convention, with tens of thousands of protesters expected to rally every day against the Biden-Harris administration's support for Israel's military offensive in Gaza.

The demonstrations are expected to begin on Sunday and continue through Thursday, with major gatherings scheduled for Monday and Wednesday in particular.

"We are ready," Chicago Mayor Brandon Johnson told ABC on Sunday, adding that his police force was working with the Secret Service and other agencies to ensure a "safe, peaceful, yet vibrant" convention.

Illinois Governor JB 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.

The four-day Democratic bonanza comes hot on the heels of one of the most tumultuous election cycles in memory, including an assassination attempt on Trump and the 81-year-old Biden's stunning withdrawal.

Trump (78) has struggled to adapt to a shake-up which has made him, rather than Biden, the oldest presidential candidate in US history.

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The Republican nominee has lashed out at Harris with increasingly bizarre personal insults despite appeals by fellow Republicans to stick to the issues.

At a rally in Pennsylvania on Saturday, the former president doubled down on his confrontational style, saying that he was "much better looking" than Harris and branding her a "lunatic."

Pennsylvania is perhaps the most prized of the half-dozen key swing states that might decide the 2024 race, and both candidates are making repeated visits.

Harris was squeezing in some crucial campaigning there with running mate Tim Walz on Sunday, launching a short bus tour from Pittsburgh to woo blue-collar voters.

All eyes will turn on Monday to the Chicago convention, which is Harris's big chance to tell her story to a public that is still getting to know the candidate.

Harris is due to speak on the final day, but CNN reported that she would also appear on stage with Biden when he gives his speech on Monday.

The aging president is reportedly still frustrated by the way Democrats pushed him out after a catastrophic debate performance against Trump in June.

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.

Biden will "make the case for Vice President Kamala Harris" and "highlight the stakes of the election for all Americans," the Harris campaign said.

Former presidents Bill Clinton and Barack Obama will also appear during the week, along with former Secretary of State Hillary Clinton and former First Lady Michelle Obama.

Taking the stage on Wednesday will be Harris's vice presidential pick Walz, the Minnesota governor who has made his name with attacks branding Trump and his running mate Vance as "weird."

While Democrats meet in Chicago, Trump will be crisscrossing the country, with rallies scheduled in Pennsylvania, Michigan, North Carolina, and Arizona throughout the week.