The parliamentary move was the latest threat to Mr. Maliki’s hold on power and reflected rising anger among rivals over his leadership, but it appeared unlikely that the law, which would need to approved by Iraq’s president, would ever go into effect.

Mr. Maliki’s coalition in Parliament boycotted the vote, and an official close to the prime minister called it unconstitutional and vowed to appeal to the federal courts, which on paper are independent but in practice bend to Mr. Maliki’s will.

Sami al-Askari, a lawmaker from Mr. Maliki’s coalition, said the law would “not see the light of day” because, he said, it is unconstitutional. “We are not worried about the vote on this law,” Mr. Askari said.