Former President Joseph R. Biden Jr. is escalating his battle against Republican claims that he might not have been in control of high-profile clemency decisions issued under his name at the end of his term and, more generally, that his cognitive state impaired his functioning in office.

In an interview with The New York Times, Mr. Biden said that he had orally granted all the pardons and commutations issued at the end of his term, calling President Trump and other Republicans “liars” for claiming his aides had used an autopen to do so without his authorization.

“I made every decision,” Mr. Biden said in a phone interview on Thursday, asserting that he had his staff use an autopen replicating his signature on the clemency warrants because “we’re talking about a whole lot of people.”

The interview was Mr. Biden’s first about the parallel investigations begun by the Trump White House, the Justice Department and Congress into a series of clemency decisions made by Mr. Biden in his final weeks in office and his mental acuity during his term.

Republicans in Congress have demanded sworn interviews with former Biden aides, prompting them to hire their own lawyers. Some lawyers are said to have warned their clients not to talk publicly and about the dangers of testifying because the Justice Department under Mr. Trump might be eager to bring perjury charges over any inconsistency, no matter how minor.

Mr. Biden’s former White House doctor, who has said his medical evaluations showed he was fit to serve, invoked the Fifth Amendment to avoid answering questions from lawmakers last week. His lawyers cited the pending Justice Department inquiry and the risk of being ensnared in ambiguous circumstances. But Mr. Biden, as a former president, has greater constitutional defenses against being subpoenaed by Congress.