NEW YORK (Reuters) - Donald Trump faces long odds in his bid to reverse his conviction on criminal charges stemming from hush money paid to a porn star despite a landmark U.S. Supreme Court ruling recognizing broad presidential immunity from prosecution, according to legal experts.

Trump's lawyers last week urged Justice Juan Merchan to set aside the jury's May 30 guilty verdict on the New York state criminal charges, citing the Supreme Court's July 1 decision that former presidents cannot be criminally prosecuted for official acts taken under their "core constitutional powers."

The ruling also stated that a former president has "at least a presumptive immunity" for "acts within the outer perimeter of his official responsibility," meaning prosecutors face a high legal bar to overcome that presumption.

Merchan delayed Trump's sentencing to Sept. 18 from July 11 in light of the defense motion. Trump was convicted in Manhattan in the first-ever criminal trial of a former U.S. president. He is the Republican candidate challenging Democratic President Joe Biden in the Nov. 5 U.S. election.

Several legal experts said Trump will have a tough time convincing the judge to overturn the conviction because much of the conduct at issue predated Trump's 2017-2021 presidency and because evidence from his time in office related to personal matters, not official acts. The Supreme Court found no immunity for unofficial acts by a president.

"Falsifying business records to pay off a porn star would not fall even within the outer stratosphere, let alone the outer perimeter of official presidential duties," Fordham University law professor Cheryl Bader said.

Trump, 78, was convicted on 34 counts of falsifying business records to cover up his reimbursement of his former lawyer Michael Cohen's $130,000 payment to adult film actress Stormy Daniels to stay quiet before the 2016 election about a sexual encounter she said she had with Trump a decade earlier. Trump has denied the encounter and has called the case politically motivated.

Prosecutors with Manhattan District Attorney Alvin Bragg's office have called the payment part of a broader scheme to influence the election by averting a sex scandal that could have influenced decisions by voters. Trump went on to defeat Democrat Hillary Clinton.

Trump has vowed to appeal the verdict after he is sentenced. The case should not even get that far, according to his lawyers. In a July 1 letter to Merchan, they said prosecutors presented jurors with evidence involving official acts as president, including social media posts he made about his payment to Cohen, an ethics form describing the reimbursement, and a conversation with a White House aide about Daniels.

Those actions cannot fairly be described as among the president's official duties, New York Law School professor Steven Cohen said.

"To the extent there were activities while he was president, they seem to be almost the definition of unofficial conduct," he added. "The likelihood that a New York court will reverse that conviction I think is remote approaching nonexistent."

Trump's lawyers declined to comment. A spokesperson for Bragg's office did not respond to a request for comment.

'HARMLESS ERROR'

There are precedents for overturning verdicts following Supreme Court decisions made after a trial's conclusion. A federal judge in Brooklyn last year threw out two convictions in a soccer bribery case in light of a May 2023 Supreme Court ruling that placed new limits on the circumstances under which a fraud law could be applied.

Even if Merchan were to find that any piece of evidence should not have been shown to the jury in light of the Supreme Court immunity decision, that would not automatically mean the judge would have to set aside the verdict, Cardozo Law School professor Gary Galperin said.

Instead, Merchan would have discretion to determine whether the evidence was crucial to the conviction, Galperin said. If the judge decides that the use of this evidence did not deprive Trump of the constitutional right to a fair trial - known in legal parlance as a "harmless error" - the conviction would likely stand, Galperin added.

"There is no perfect trial," said Galperin, a former prosecutor in the Manhattan District Attorney's office. "The justice system tolerates imperfection."

Defense lawyers are due to lay out their arguments for setting aside the verdict more fully in a court filing by Wednesday, and prosecutors would then have until July 24 to respond. Merchan has said he will decide by Sept. 6, and that if the conviction stands the judge would sentence Trump on Sept. 18.

(Reporting by Luc Cohen in New York; Editing by Will Dunham)

By Luc Cohen