President Joe Biden, Democratic superstar Barack Obama and Republican firebrand Donald Trump all converged Saturday on Pennsylvania to push their parties to the finishing line in a race Biden said marks a "defining" moment for US democracy.

The battle of the serving and two former presidents marked the start of a final crescendo before Tuesday when Americans will decide who controls Congress during the last two years of Biden's first term.

Polls put Republicans well ahead in the fight for the House of Representatives and also show them gaining momentum in the Senate races as voters, riled up by culture wars around gay rights and abortion, seek to take out frustration over four-decades-high inflation and rising illegal immigration.

With Pennsylvania one of the handful of swing states that will decide the overall balance of power, both sides brought out their big guns -- and the contrasts were dramatic.

Biden and Obama rallied in Philadelphia alongside Senate hopeful John Fetterman and governor candidate Josh Shapiro.

Trump -- who was defeated by Biden in 2020 but has spent the interval promoting conspiracy theories and plotting a possible White House comeback -- flew to Latrobe to boost Fetterman's opponent, TV celebrity doctor Mehmet Oz, and Shapiro's far-right opponent Doug Mastriano.

Speaking to thousands in a Philadelphia arena, Biden and the Democratic candidates labeled the Republicans as the party of the wealthy and emphasized their support for trade unions, social security and abortion access.

Voters face "a choice between two vastly different visions of America," Biden said.

But citing Trump Republicans' growing support for conspiracy theories, Biden said an even bigger agenda is at stake.

"Democracy is literally on the ballot. This is a defining moment for the nation and we all, we all must speak with one voice," Biden said.

In a rambling speech, Trump claimed the country is run by "communists" and repeatedly said that his attempts to overturn the 2020 election were justified, before urging Republicans to deliver "a humiliating rebuke."

"If you want to stop the destruction of our country and save the 'American Dream,' then this Tuesday you must vote Republican in a giant red wave," he said.

 - Democratic star -

Obama, who had also addressed an earlier rally in Pittsburgh, got the loudest cheers of the night in Philadelphia, repeatedly urging supporters to make sure they vote.

"A lot of folks don't pay a ton of attention to politics the way they do in a presidential year. Maybe they don't think Congress matters as much. Maybe they don't think their vote will matter," he said.

But "fundamental rights..., reason and decency are on the ballot," he said, attacking Republicans as increasingly averse to everything from science to respect for rules.

"Democracy itself is on the ballot. The stakes are high," Obama said in an echo of Biden's warning, his voice going hoarse.

Still the party's most bankable star six years after leaving the White House, Obama hopes his support will give Fetterman the crucial extra shove.

Although Fetterman faces the added challenge of recovering from a serious stroke, he and Oz are in a dead heat.

 - Trump's comeback bid -

In Latrobe, Trump was tapping into support from a working class region that delivered him big margins in the 2016 and 2020 presidential elections.

Success for candidates he supports on Tuesday would help Trump launch his own comeback campaign, despite facing serious legal threats over attempts to overturn his 2020 defeat and the hoarding of top secret documents from the White House at his Florida golf resort.

In a speech laden with immigrant baiting, lies about supposed election fraud, and lurid claims that he and his supporters are victims of a "police state," Trump also continued to drop hints that he will soon declare a new presidential run.

In a "very, very, very short period" his fans will be "so happy," he said.

One supporter in Latrobe, Shawn Ecker, 44, voiced excitement about a possible Trump candidacy for 2024 "because we need our country back. We really do. And it's not going to happen if someone doesn't stand up like he is."