Legal experts and multiple credible sources confirm that a former president cannot unilaterally declare another president's pardons null and void, and there is no constitutional or legal basis for President Trump to invalidate President Biden's pardons.
Comer on Biden: "President Trump can declare those pardons and executive orders null and void and I think that would stand up in a court of law.

Can Donald Trump Legally Void Joe Biden's Pardons? What To Know - Newsweek
Source: newsweek.com
Key Evidence
The Constitution does not provide any mechanism for a former president to revoke or invalidate pardons issued by a successor.
nytimes.comnytimes.comTrump Says Biden’s Pardons are ‘Void’ and ‘Vacant’ Because of Autopen
pbs.orgpbs.orgFact-checking Trump's claim that Biden pardons are 'void' because he used an...
Legal experts affirm that the use of an autopen for signing pardons is valid and customary government practice.
politifact.compolitifact.comPresident Donald Trump claimed that former President Joe Biden’s pardons are...
Courts have consistently held that presidential pardons are final acts not subject to reversal by subsequent presidents.
msnbc.commsnbc.comTrump can’t ‘VOID’ Biden’s pardons. But he can send a chilling message
What the Evidence Shows
The claim attributed to Comer that President Trump can declare Biden's pardons and executive orders null and void and that such a declaration would stand up in court is contradicted by established constitutional law and expert legal analysis.
Multiple sources, including The New York Times, Politifact, PBS NewsHour, and MSNBC, explain that presidential pardons are constitutionally protected acts that cannot be undone or invalidated by a subsequent president. The Constitution grants the pardon power exclusively to the sitting president, with no provision for reversal by successors.nytimes.comnytimes.comTrump Says Biden’s Pardons are ‘Void’ and ‘Vacant’ Because of Autopen
politifact.compolitifact.comPresident Donald Trump claimed that former President Joe Biden’s pardons are...+2 more
The controversy over Biden's use of an autopen to sign pardons was cited by Trump as a reason to declare them void; however, legal experts and official memoranda confirm that the use of an autopen is valid and customary in government practice, with no legal precedent invalidating such pardons.pbs.orgpbs.orgFact-checking Trump's claim that Biden pardons are 'void' because he used an...+2 more
Newsweek and TIME report that while Trump publicly claimed the pardons were "null and void," these claims lack legal standing. MSNBC further clarifies that although a former president cannot void pardons, political or retaliatory actions outside the law may be attempted but would likely be blocked in court.msnbc.commsnbc.comTrump can’t ‘VOID’ Biden’s pardons. But he can send a chilling message
newsweek.comnewsweek.comCan Donald Trump Legally Void Joe Biden's Pardons? What To Know+1 more
Therefore, the assertion that Trump's declaration would hold up in a court of law is unsupported by constitutional law, judicial precedent, or expert opinion. The claim is legally baseless and misleading.
In summary, the evidence strongly indicates that presidential pardons issued by Joe Biden remain valid and cannot be nullified by Donald Trump or any other former president.