Biden, Trump push US Senate candidates

US President Joe Biden has told voters in Pennsylvania that a Democratic loss in Tuesday's midterm elections would have "decades" of consequences, while Republican candidates for Congress predicted a sweeping victory.

The biggest names in US politics, including Democrats Biden and former president Barack Obama, as well as Republican former president Donald Trump, were in Pennsylvania on Saturday hoping to tip the balance in a pivotal midterm Senate race between Democratic Lieutenant Governor John Fetterman and Republican celebrity doctor Mehmet Oz.

"Folks, three days, three days until one of the most important elections in our lifetime. The outcome is going to shape our country for decades to come, and the power to shape that outcome is in your hands," Biden said.

"It's a choice. A choice between two vastly different visions of America."

Trump, who sources say is preparing to launch a third consecutive run for the White House after the midterms, continues to falsely claim that his 2020 defeat by Biden was the result of widespread fraud. Multiple courts, state agencies and members of his own administration rejected that claim as untrue.

Still, opinion polls show a significant number of Republican voters accept the claim, as do many candidates for Congress, governor and state offices overseeing election administration.

Speaking before Trump at a rally in Latrobe, Pennsylvania, Representative Glenn Thompson told supporters that Republicans were headed for major gains on Tuesday.

"Who's got a surfboard?," he asked. "There's gonna be a red wave."

Speaking to supporters in downtown Pittsburgh, Obama warned the politically motivated attack last week on Democratic House of Representatives Speaker Nancy Pelosi's husband, Paul, was a product of hateful rhetoric.

"This habit we have of demonising political opponents, of saying crazy stuff, it creates a dangerous climate," Obama said, without referencing Republicans directly.

Republicans contend that Democrats have also engaged in political violence, citing the widespread anti-racism protests that rocked the country in 2020.

On Sunday, they criticised Democrats for failing to keep their focus on inflation and crime, two of voters' principal concerns, according to most polls.

Trump is set to drum up support for his handpicked Republican Senate nominee, Oz, and Republican gubernatorial nominee Doug Mastriano at a rally in Latrobe, southeast of Pittsburgh.

He is also working to maintain his own profile as he contemplates another White House run..

That could set the stage for a Biden-Trump rematch, though some Democrats say heavy losses for Biden's party on Tuesday could increase pressure on the president to step aside and let someone else carry the party's mantle in 2024.

The Fetterman-Oz Senate race is one of three critical contests, along with Georgia and Nevada, that will determine whether Democrats hold onto their razor-thin majority in the Senate, and with it the power to confirm Biden's nominees to posts ranging from his Cabinet to the Supreme Court.

Nonpartisan election forecasters and polls show Republicans are heavy favourites to win control of the House, with the Senate a toss-up. Control of even one would give Republicans the power to block Biden's legislative agenda and launch potentially damaging investigations.

Election officials have warned that it could take days after Tuesday for final results to be clear in closely contested elections, such as the Pennsylvania and Georgia Senate races.