Donald Trump goes to court Monday as the first US ex-president ever to be criminally prosecuted, a seismic moment for the United States as the presumptive Republican nominee campaigns to re-take the White House.

The scandal-plagued 77-year-old is accused of falsifying business records in a scheme to cover up an alleged sexual encounter with adult film actress Stormy Daniels to shield his 2016 election campaign from adverse publicity.

The so-called hush money affair is only one of four criminal cases hanging over Trump and some legal observers suggest it is the least serious.

But the real prospect of Trump becoming a convicted felon -- and potentially facing jail time -- throws an unprecedented wild card into an already unpredictable election.

The right-wing populist is running on dark vows of "vengeance" against Democratic President Joe Biden, who beat him in 2020.

Trump has said he will take the stand in the case, which gets underway from 1330 GMT, an unusual and often risky move for defendants.

"I've been indicted more than Al Capone, the great gangster," Trump said to cheering supporters at a rally Saturday in key swing state Pennsylvania.

"There is absolutely nothing here," he insisted. "There is no misdemeanor, there is no felony, there's no federal crime."

But ahead of any testimony by Trump, the trial will start Monday with a likely lengthy and contentious process to select 12 jurors and their alternates.

- Trump alleges 'lawfare' -

The pool of ordinary citizens convened by Judge Juan Merchan must answer a questionnaire including checks on whether they have been members of far-right groups.

The actual charges revolve around highly technical finance laws.

Trump is accused of illegally covering up remittances to his longtime attorney and fixer Michael Cohen, who was using the funds to pay Stormy Daniels to keep quiet about the alleged sexual encounter in the final weeks of the 2016 election campaign.

A New York grand jury indicted Trump in March 2023 over the payments made to Daniels, whose real name is Stephanie Clifford, with the ex-president charged with 34 counts of falsifying business records.

He denies the charges and could use the trial, scheduled for up to two months, as a prominent platform to decry what he alleges is "lawfare" and election interference by his political opponents.

Trump also claims he will not get a fair trial in heavily Democratic New York.

However, the real estate magnate and longtime reality TV show star is using the limelight as an unlikely campaign boost -- touting himself as a victim and using supporter outrage to fundraise.

Even if convicted, he would be able to appeal and would not be barred from continuing to run, or even being elected president on November 5.

- Influence on election -

Trump's other three criminal cases -- centered on his alleged hoarding of top-secret documents in Florida after he left the White House and his involvement in attempts to overturn the 2020 election -- all face multiple delays.

In the New York case, Trump has repeatedly failed to secure meaningful delays and Merchan has signaled he will run the trial with a firm hand.

Last week the judge extended an existing gag order, in place to prevent Trump from attacking those involved in the trial, widening it to cover family members of the judge and Manhattan District Attorney Alvin Bragg, the lead prosecutor.

The expansion of the order came after Trump lashed out at Merchan and his daughter in a series of posts on Truth Social.

"The American people and many voters do consider important whether a jury concludes that Trump is guilty of engaging in criminal misconduct," said University of Richmond law professor Carl Tobias.

"This finding might influence how they vote this November."

In New York, where Trump has been a fixture for decades as an entrepreneur, celebrity playboy, politician and now criminal defendant, there is little sympathy.

"I don't know if he'll get a fair trial, but whatever happens, he caused it on himself," said caregiver Alberto Vasquez, 45.