He jabs at his Democratic foes: Cory Booker the "disaster" mayor, Elizabeth Warren the "Pocahontas" pretender and "sleepy" Joe Biden.

"I want to be careful," Trump tells the crowd, feigning a confession. He doesn't want to hit his potential challengers too badly, he says, because then the Democrats may find "somebody that's actually good to run against me. That would not be good."

The venue may be Council Bluffs, Iowa, or Erie, Pennsylvania, or Topeka, Kansas, but the formula is largely the same.

Start with a few derisive nicknames, mix in some dreamy-eyed reminiscences of Election Night 2016, spice things up with an unexpected quip or zinger out of left field and you've got Trump's recipe for a successful campaign rally.

Trump's rallies once were the cornerstone of an unconventional, star-powered presidential campaign that eschewed traditional organising and defied every expectation.

Now they're being deployed with gusto as Trump and his team work frantically to defy polls and precedent and save his Republican majority in Congress in November's midterm elections.

The rallies - more than two dozen so far to boost GOP candidates - never fail to delight Trump's supporters.

"Look at this," says Brenda McDonald, 58, of Woodbury, Minnesota, gesturing to the thousands of people standing ahead of and behind her in a line that wound around buildings and snaked through alleys for at least a mile when Trump's rally tour stopped in her state on October 4.

"Have you ever seen rallies like this before?" she asked.

Trump has been aggressively campaigning across the county to try to boost vulnerable Republicans before the November 6 elections, when the stakes couldn't be higher.

A Democratic takeover of Congress would stymie his agenda and mire his administration in endless investigations, including possible impeachment proceedings.

Trump's team believes his appearances fire up his loyal base, countering the wave of Democratic enthusiasm that polls suggest will lead to significant Democratic gains, especially in the House.

But after more than 350 rallies since he first began his presidential run, some things have changed.

Trump's supporters remain as enthusiastic as ever, standing for hours in hot sun or driving rain and exploding into thundering applause when he takes the stage.

They wave the same signs, wear the same hats, and chant the same "Build that wall!" and "Lock her up!" refrains that they did during the early days of Trump's campaign.

But the once insurgent candidate, who told his supporters the system was rigged against them, is now president. And he's been delivering on many of his campaign promises, in spite of lackluster approval ratings.

Trump's 2016 rallies had the feel of angry, raucous, grievance sessions, as Trump's "deplorables" gathered in the face of charges they were racist, bigoted and could never win.

Gone now is the darkness, the crackling energy, the fear of potential violence as supporters and protesters faced off, sometimes trading blows.

The mood now is calmer, happier, more celebratory.

Trump's rallies have gone mainstream, complete with a new playlist featuring Rihanna, Macho Man by the Village People and Prince's Purple Rain.

Trump's campaign, which was notably stingy during his own election effort, has been investing heavily in his recent tour, covering all the costs of organising and paying for the rallies, including footing the Air Force One bills, according to the campaign.

Trump's crowds seem most entertained when he veers into offensive, "politically incorrect" territory.

He's bragged about how easily he could pummel Biden, the former vice president, or Booker, the New Jersey senator and former Newark mayor, or Warren, the Massachusetts senator whom Trump denigrates for her claims of Native American heritage.

And he's mocked the Senate testimony of California professor Christine Blasey Ford, who accused Kavanaugh of sexually assaulting her in high school.

Those moments add spontaneity and a tinge of sinister mischief that keep Trump's speeches interesting, even as they grow increasingly formulaic.

Indeed, the rallies, at times, take on the feel of a high school reunion, with Trump taking the role of star football jock, reliving his glory days, play by play.

In laborious detail, Trump takes his audience through Election Night 2016, re-enacting cable news anchors calling state after state in his favour, adding dramatic commentary.

"Was that the most exciting evening of our lives?" Trump asked his crowd in Erie, Pennsylvania, on Wednesday. "Was that the most exciting night? Was that the greatest?"

Australian Associated Press