When Smith handed down his charges in August 2023, a majority of Americans (51%) thought the indictment was very serious, according to an ABC News/Ipsos poll taken shortly after. Overall, 65% of adults thought then that the charges were serious, including 51% who said they were very serious and 14% who said they were somewhat serious.

At the time, a plurality of Americans (49%) said Trump should suspend his presidential campaign, while 36% said he shouldn't.

On Trump’s immunity claim, an overwhelming number of Americans (66%) thought in that poll that the former president should not be immune to criminal prosecution for actions he took while president, according to an ABC News/Ipsos February poll. That included 45% of Republicans and 83% of Democrats. Independents fell in between at 66%.

As Smith's team has noted in arguing against Trump, the former president's lawyers during his second Senate impeachment trial said in very clear terms that they believed a president could in fact be criminally prosecuted by the Department of Justice -- the opposite of what his legal team is currently arguing.

"If my colleagues on this side of the chamber actually think that President Trump committed a criminal offense -- and let's understand, a high crime is a felony and a misdemeanor is a misdemeanor; the words haven't changed that much over time -- after he is out of office, you go and arrest him," one of his attorneys said in opening statements during his impeachment trial in 2021, shortly after Jan. 6.

"We have a judicial process in this country, we have an investigative process in this country, to which no former officeholder is immune,” Trump's attorney said then.

But now, facing federal charges related to Jan. 6, Trump's lawyers argue a president can only be prosecuted if he is impeached, convicted by the Senate and removed from office.

The Senate in his second impeachment trial acquitted, though a majority of lawmakers voted to convict him.

-ABC News' Katherine Faulders

Trump appeared confident while addressing the presidential immunity hearing during remarks Thursday morning in Manhattan.

"We have a big case today -- this judge wouldn't allow me to go, but we have a big case today at the Supreme Court on presidential immunity, a president has to have immunity," Trump said at a construction site where he met with union members, workers and supporters. "If you don't have immunity, you just have a ceremonial president," he insisted.

When asked about the justices, three of whom he named to the high court, Trump said, "They're very talented people, they're very smart, they know what they're doing."

Trump stopped by the midtown Manhattan site, where the new JPMorgan headquarters is under construction, before heading to the courthouse for his hush money trial. He denies all wrongdoing.

-ABC News' Lalee Ibssa, Soo Rin Kim and Mike Pappano

Trump, seeking to quash the federal election subversion case brought against him by Smith last year, is claiming he has “absolute” immunity from the criminal prosecution for anything related to his presidency.

Two lower courts have rejected that argument.

First, District Judge Tanya Chutkan (who is overseeing the trial) determined that someone once serving as president “does not confer a lifelong 'get-out-of-jail-free' pass.”

Then, a three-judge panel on the U.S. Court of Appeals for the District Court of Columbia unanimously struck down Trump's claims. They warned his view, if accepted, would “collapse” the country's political system.

Trump then appealed to the U.S. Supreme Court. His trial is on hold while this matter plays out in the courts.

Trump's attorney, John Sauer, addressed the justices first, saying in his opening statement that unless presidents are granted the absolute immunity that Trump seeks, "Every current president will face de facto blackmail and extortion by his political rivals while he is still in office."

Sauer also called out controversial policies of other commanders in chief, such as George W. Bush's regarding the Iraq War and a Barack Obama-approved drone strike against a U.S. citizen as part of anti-terrorism operations overseas, while saying the implications of the court's decision here "extend far beyond the facts of this case."