Others said Mr. Starr would be best suited to explain the principled differences because of his experience. “Ken is an excellent choice to help the president,” said Robert J. Bittman, a former deputy. “Ken is one of the few lawyers who has extensively studied and experienced the precedents and nuances of the impeachment process. He will be a valuable resource to the president and the Senate.”

For a time in the 1990s, Mr. Starr, 73, was a household name, the prosecutor pursuing Mr. Clinton first over the Whitewater land deal and then over the president’s efforts to thwart a sexual harassment lawsuit by covering up an affair with a White House intern. To his admirers, Mr. Starr was an upright pursuer of a lying, philandering president who had dishonored the Oval Office. To his critics, Mr. Starr was a moralistic, sex-obsessed Inspector Javert persecuting a president out of ideological animus.

Mr. Starr’s investigation confirmed that Mr. Clinton had a sexual relationship with Monica S. Lewinsky despite the president’s denials under oath and efforts to coach other potential witnesses to hide his indiscretions during a lawsuit brought by Paula Corbin Jones, a former Arkansas state worker who accused him of sexual harassment when he was governor.

Acting on Mr. Starr’s findings, the House impeached Mr. Clinton in December 1998, largely along party lines, but the Senate acquitted him in February 1999, concluding that the president’s wrongdoing did not justify removing him from office. Mr. Starr testified in the House but played no direct role in the Senate trial.

Mr. Clinton was separately found in contempt of court and fined by a federal judge and later struck a deal with Mr. Starr’s successor in which the president admitted not telling the truth under oath, paid a fine and surrendered his law license.

Mr. Starr, who served as a federal appeals court judge and then as President George Bush’s solicitor general, was once a legal star among Republicans and seen as a possible Supreme Court justice. But his time as independent counsel made him politically radioactive.

He went on to serve as dean of the Pepperdine University Law School and as president of Baylor University but was demoted and later resigned from Baylor after an investigation found the university mishandled accusations of sexual assault against members of the football team. The investigators rebuked the university leadership, saying that it had “created a perception that football was above the rules.”