Today, Trump’s lawyers will try to “defend the indefensible… and they may just succeed,” Albany Law School's Ray Brescia tells the BBC.

The professor, who is of the opinion that the court should rule against Trump’s immunity claim, says he is “deeply concerned” that the justices may find a way to “stymie the prosecution” by adding procedural roadblocks, thus delaying the criminal trials further.

Of course, there is a possibility that the court may rule in Trump’s favour, agreeing that he has full immunity - “but there really are simply no legitimate grounds” for this outcome, Brescia says.

In January, Trump laid out his legal case for reporters after an immunity hearing was held at the Supreme Court.

“I feel that as a president you have to have immunity, it’s very simple,” he said.

He said that he was ”working for the country” when he argued that the November 2020 election was unfairly won by Biden, and that he was simply investigating “voter fraud”.

Investigators have never uncovered any evidence of widespread voter fraud in the 2020 presidential election.

In August 2023, the justice department’s special prosecutor Jack Smith charged Trump with four counts of criminally conspiring to overturn the 2020 election.

Trump pleaded not guilty, and in October, his lawyers filed to dismiss the charges, claiming that their client enjoyed absolute presidential immunity.

In December, US District Judge Tanya Chutkan rejected Trump’s immunity claims, ruling that having served as president “does not confer a lifelong 'get-out-of-jail-free' pass”.

Trump quickly appealed the decision and Smith asked the Supreme Court to take up the case, expediting the appeals process.

The Supreme Court denied the fast-track request, and a three-judge panel in a Washington DC court unanimously ruled in February against Trump’s immunity claim.

By the end of February, the Supreme Court agreed to hear the case.

Anthony Zurcher
BBC North America correspondent

This case presents a chance for the US Supreme Court to break new legal ground.

It has never before considered whether former president have immunity from criminal prosecution for actions taken while they were in office.

Donald Trump’s lawyers argue that this is an important protection, allowing chief executives to take action without fear of being harassed by opponents in positions of legal authority after they leave office.

Jack Smith’s special counsel team counters that no one – not even former presidents – should be above the law.

The Supreme Court, in deciding this case, is likely to balance these two competing concerns. How the justices end up ruling will set the standard not just for Trump, but for every future US president.

Read Anthony's full story on today's hearing here.

The question in this landmark case is relatively short and easy to understand - but the answer will set a powerful precedent for US presidents.

The Supreme Court ruled in 1982 that US presidents cannot be sued for what they do during their official duties - but that is civil action.

Now, for the first time, it will consider if that immunity extends to criminal prosecution.

Presidential immunity protects a sitting president from civil lawsuits relating to official duties while they’re in office - basically, a president can’t be sued for doing their job.

It’s not enshrined in the US constitution, it’s a legal doctrine.

But that doesn’t mean the president is above the law - it doesn’t cover them for unofficial conduct, criminal behaviour, or actions before they were president.

Donald Trump is now facing dozens of criminal charges across four cases - becoming the first president in US history to be criminally charged.

So, depending on what the Supreme Court justices decide, this case could set a crucial precedent in the way US presidents are held accountable for abusing their executive power.

Hello, it’s a big day at the US Supreme Court - both for Donald Trump and for the future of how presidents may be held accountable for alleged criminal behaviour.

In about an hour, the court will begin to hear opening arguments into whether Trump has sweeping immunity from criminal prosecution.

Their verdict, which we may not get for a few weeks at least, will be one of the most important decisions on presidential power in decades.

If the justices rule that Trump has presidential immunity, several of the criminal charges he’s facing, including that he allegedly conspired to overturn the 2020 election result, will be dropped.

Lower courts have already rejected his immunity argument, but his legal team appealed to the Supreme Court, which agreed to hear it.

Stick with us as we explain what’s going on and why it matters so greatly.