WASHINGTON — Prominent conservative lawyers joined liberal colleagues Tuesday in opposing Bush administration anti-terror tactics, arguing that an immigrant held as an enemy combatant has a right to seek his freedom in court.

The legal brief, filed in the case of suspected Al Qaeda sleeper agent Ali Saleh Kahlah al-Marri, argues that a new military commissions law is unconstitutional.

The argument has been made in this and other detainee cases, but Tuesday’s brief is notable for the bedfellows created by the politics of anti-terrorism. Staunchly Democratic law school deans Harold Koh of Yale and Laurence Tribe of Harvard were joined by lawyers such as Steven Calabresi, who served in the Reagan and first Bush administrations and helped found the conservative Federalist Society.

“It shows the phrases ‘conservative’ and ‘libertarian’ have less overlap than ever before,” a University of Chicago law professor and Federalist Society member who signed the brief, Richard Epstein, said. “This administration has lost all libertarians on all counts.”

In June, the Supreme Court said the Bush administration’s handling of detainees violated American and international law. Mr. Bush then pressed for, and got, a new law that he said would help the government prosecute terrorists.

The Military Commissions Act allows the military to hold detainees indefinitely and strips them of the right to challenge their imprisonment in American courts. The Justice Department defends the law as a constitutional and necessary tool to combat terrorism.

Civil rights groups and conservatives with a libertarian viewpoint see the law as a government infringement on personal freedom.

“This involves the executive branch changing the rules to avoid challenges to its own authority,” Mr. Koh said Tuesday.