Wilkes-Barre, Pennsylvania - Donald Trump launched fresh personal attacks against White House rival Kamala Harris on Saturday, as new polling showed her making major gains in key battleground states ahead of next week's Democratic National Convention.

Speaking at a rally in Pennsylvania, Trump began by blaming Vice President Harris for unleashing "devastating" inflation – one of the biggest issues of the campaign – but he soon drifted off script, mocking Harris's laugh and calling her a "communist" and a "lunatic."

At one point, criticizing a portrait of Harris on the cover of Time magazine, Trump insisted he was "much better looking than her."

Republicans and Trump advisers – concerned by Harris's energized campaign – have publicly urged him to stick to the issues and lay off the personal attacks, which they believe play badly with the undecided and independent voters he needs to win the November 5 election.

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But the former president has shown no sign of changing his populist, confrontational style.

"You don't mind if I go off teleprompter for a second, do you?" he asked the crowd about 15 minutes into his speech, before proceeding to reel off a now familiar list of insults at Harris.

"People say, please don't use bad language. Please don't call people stupid," Trump said. "Please, don't call her a lunatic. And I said, but that's what she is, she's a lunatic."

The momentum in the White House race has shifted dramatically since President Joe Biden abruptly pulled out on July 21, with Harris's whirlwind entry enthusing the Democratic Party base.

A survey by The New York Times and Siena College published Saturday had Harris storming back into contention in four critical battleground states that Trump had looked set to win comfortably against Biden.

The poll will likely trigger further consternation in Trump's campaign team, with the vice president now ahead in Arizona and North Carolina, and getting closer in Nevada and Georgia.

At the rally Saturday in Wilkes-Barre, Trump skewered Harris on her historic opposition to fracking – an unpopular stance in Pennsylvania, which is the second-largest natural gas-producing US state after Texas.

But he spent far longer reviewing his debate performance against Biden back in June and on meandering anecdotes about everyone from Italian screen legend Sophia Loren to French President Emmanuel Macron.

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With polls showing the head-to-head race very close, it is the swing states – especially Pennsylvania – that will decide the final result under the US electoral college system.

Trump lost the state by a narrow margin against Biden in 2020 but has strong support in rural areas and small towns.

Harris will be in Pennsylvania on Sunday, making several stops on her campaign bus near Pittsburgh before heading to Chicago for the Democratic National Convention that opens on Monday.

She will be hoping to sustain her poll momentum at the four-day event which will include keynote speeches from party leaders like Biden and former president Barack Obama.

Harris will round out the proceedings on Thursday evening with her own speech to formally accept the party nomination.