Supreme Court poised to rule in Trump presidential immunity case on Monday

The U.S. Supreme Court appears to be saving its most consequential ruling this term for last.

When the justices meet on Monday for a final day of opinions, they are expected to issue a blockbuster decision on whether a former president is shielded from criminal liability for "official acts" taken while in the White House.

In the case, Donald Trump is claiming such immunity to try to quash the federal election subversion prosecution brought by special counsel Jack Smith.

Smith charged Trump with four felony counts, including conspiracy to defraud the United States and conspiracy to obstruct an official proceeding, over his efforts to hold onto power after his 2020 election loss. Trump pleaded not guilty and has denied any wrongdoing.

The trial was set to start March 4 but has been delayed while the high court considers the immunity question.

Lower courts have flatly rejected Trump’s arguments.

U.S. District Judge Tanya Chutkan, who is overseeing Trump's election subversion case, said whatever immunities a sitting president may enjoy, the position "does not confer a lifelong 'get-out-of-jail-free' pass."

And a three-judge panel in the U.S. Court of Appeals later unanimously rejected Trump's claims, warning if they were to be accepted they would “collapse our system of separated powers.”

While the Supreme Court did not appear on board with Trump's more sweeping claim of "absolute" immunity, several justices appeared open to some level of protection for former presidents when they heard oral arguments in late April -- months after Smith first asked the court to intervene on the issue.

Their questioning largely focused on what presidential acts would be protected and which would not.

Justice Elena Kagan pressed Trump's attorney if it would mean a former president could escape criminal liability for ordering a coup or selling nuclear secrets. Justices Clarence Thomas and Samuel Alito wondered if past presidents who oversaw controversial policies like the internment of Japanese-Americans during World War II or Operation Mongoose would have been prosecuted after they left office.

What the justices decide on the immunity issue will set a new standard for presidential power, and will affect whether Trump stands trial for his unprecedented actions in the aftermath of the 2020 election.

"We are writing a rule for the ages," Justice Neil Gorsuch said during arguments.

ABC News' Devin Dwyer contributed to this report.