Joe Biden has said the historic ruling that former presidents have immunity violates the principle “there are no kings in America” and means the commander-in-chief can now “ignore the law”.
He was speaking after the US Supreme Court decided immunity exists from criminal prosecution for official acts while in office, but not for unofficial ones.
“Each of us is equal before the law. No one is above the law, not even the president of the United States,” Mr Biden said.
The ruling is a victory for Donald Trump, who is accused of illegally trying to overturn his 2020 election loss.
“I know I will respect the limits of presidential power as I have for the three-and-a-half years,” Mr Biden added.
“But any president, including Donald Trump, will now be free to ignore the law.”
The Supreme Court did not rule on the merits of Trump’s case, but referred it back to a lower court to decide how to apply the ruling.
It must now decide whether Trump was acting officially or privately in relation to the charges.
President Biden said Monday’s ruling also meant the ex-president was now “highly unlikely” to go on trial before US voters have their say again in four months’ time.
“It’s a terrible disservice to the people in this nation,” he said.
If Trump becomes president again in November, he may be able to use his powers to dismiss the charges against him.
He earlier celebrated the ruling, posting online: “BIG WIN FOR OUR CONSTITUTION AND DEMOCRACY. PROUD TO BE AN AMERICAN!”
The three liberal justices all dissented with the majority opinion – with Sonia Sotomayor warning it was a dangerous step for democracy.
She said it made a “mockery” of the principle that “no man is above the law”.
“In every use of official power, the president is now a king above the law,” she wrote.
The chief justice, John Roberts, insisted that wasn’t true but said they have “at least presumptive immunity from prosecution” for official acts.
The decision passed with the help of the three conservative judges Trump appointed when he was president.
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Biden v Trump: Highlights from the TV debate
Mr Biden’s comments follow his poor showing in the TV debate with Trump last week, a performance that prompted speculation the Democrats might try to replace him as candidate.
The 81-year’s remarks were his first from the White House since then, and he put in a far more assured and coherent performance – even sporting a glowing tan.
However, Mr Biden was reading from an autocue – something he did not have the benefit of during his stumbling face off with Trump.
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Mr Biden admitted last week his debating skills were not what they once were but said it was his substance and actions that mattered.
His family have also urged him to carry on.
Democrat insiders spoke of panic after the TV debacle, but any effort to force him to withdraw against his will is extremely unlikely – with the only realistic route being if he stepped down himself.