SAN FRANCISCO — The lawyer and parents of American-born Taliban soldier John Walker Lindh asked President Bush yesterday to commute his 20-year prison term, citing the case of an Australian man who was sentenced to less than a year for aiding terrorism.

Lindh, 26, was captured in Afghanistan in November 2001 by American forces sent to topple the Taliban after the September 11, 2001, attacks.

He was charged with conspiring to kill Americans and support terrorists, but he pleaded guilty to lesser offenses, including carrying explosives for the now-defunct Taliban government.

Lindh’s lawyer and father said the lighter sentence given to Australian David Hicks should be reflected in Lindh’s case.

“It is a question of proportionality. It is a question of fairness, and it is a question of the religious experience John Walker Lindh had,” attorney James Brosnahan said. “And it was not in any way directed at the United States.”

Lindh converted to Islam and went to Afghanistan to fight for the Taliban against the Northern Alliance, which received American backing.

On Saturday, Hicks pleaded guilty to supporting terrorism and acknowledged aiding Al Qaeda during the American-led invasion of Afghanistan.

He was sentenced to nine months in prison. After spending five years at the American naval base in Cuba, the 31-year-old former kangaroo skinner is likely to be transferred to a prison in Australia within weeks.

Mr. Brosnahan brokered Lindh’s plea deal and said it was the best that he could do in the political climate immediately after the 2001 attacks.

“In the atmosphere of the time, the best John could get was a plea bargain and a 20-year sentence,” said Lindh’s father, Frank Lindh. “We love our son very much. He was wrongly accused when he was found in Afghanistan.”

The White House referred telephone calls to the Justice Department, which declined to comment because it had not received Lindh’s petition.