President Biden moved on Monday to guard some of President-elect Donald J. Trump’s most high-profile adversaries against a promised campaign of “retribution” by issuing pre-emptive pardons that would make it harder if not impossible for the next administration to prosecute them.

Among those receiving pre-emptive pardons were Gen. Mark A. Milley, the former chairman of the Joint Chiefs of Staff; Dr. Anthony S. Fauci, the longtime government scientist who advised Mr. Trump during the coronavirus pandemic; and all the members of the bipartisan House committee that investigated the Jan. 6, 2021, attack on the Capitol, including former Representative Liz Cheney, a Republican from Wyoming.

“These are exceptional circumstances, and I cannot in good conscience do nothing,” Mr. Biden said in a statement. “Baseless and politically motivated investigations wreak havoc on the lives, safety and financial security of targeted individuals and their families.

“Even when individuals have done nothing wrong — and in fact have done the right thing — and will ultimately be exonerated, the mere fact of being investigated or prosecuted can irreparably damage reputations and finances,” he added.

Acting just hours before leaving office, Mr. Biden effectively turned the president’s constitutional power of forgiveness into a protective shield against what he maintains would be politically driven vengeance. No other president has employed executive clemency in such a broad and overt way to thwart a successor he believes would abuse his power.

As Mr. Biden has considered such a move in recent days, his aides have stressed that he does not believe the people who would receive pardons have actually committed any crimes. But he and his aides have cited Mr. Trump’s repeated vows to go after them upon taking office on Jan. 20.

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