President Biden fully and unconditionally pardoned his son Hunter on Sunday night, using the power of his office to wave aside years of legal troubles, including a federal conviction for illegally buying a gun, and Republican attacks that hounded the Biden family throughout the last four years.

In a statement issued by the White House, Mr. Biden said he had decided to issue the executive grant of clemency for his son “for those offenses against the United States which he has committed or may have committed or taken part in during the period from Jan. 1, 2014, through Dec. 1, 2024.”

He said he did so because the charges against his son were politically motivated and designed to hurt the president politically.

“The charges in his cases came about only after several of my political opponents in Congress instigated them to attack me and oppose my election,” Mr. Biden said in the statement. “No reasonable person who looks at the facts of Hunter’s cases can reach any other conclusion than Hunter was singled out only because he is my son — and that is wrong.”

He added: “There has been an effort to break Hunter — who has been five and a half years sober, even in the face of unrelenting attacks and selective prosecution. In trying to break Hunter, they’ve tried to break me — and there’s no reason to believe it will stop here. Enough is enough.”

Many of the president’s allies and critics had expected him to use the unique authority vested only in his office, even though the president’s spokeswoman had denied for months that Mr. Biden had any intention of doing so. NBC News first reported Sunday evening that Mr. Biden had in fact decided to pardon his son.