BAGHDAD, Iraq, Oct. 3 - Iraq's Shiite and Kurdish leaders quietly adopted new rules over the weekend that will make it virtually impossible for the constitution to fail in the upcoming national referendum, prompting Sunni Arabs and a range of independent political figures to complain that the vote was being fixed.

Some Sunni leaders who have been organizing a campaign to vote down the document said today that they might now boycott the Oct. 15 referendum, because the rule change made their efforts futile. Other political leaders also reacted angrily, saying the change would seriously damage the vote's credibility in Iraq and abroad.

Under the new rules, the constitution will fail only if two-thirds of all registered voters - rather than two-thirds of all those actually casting ballots - reject it in at least 3 of Iraq's 18 provinces.

The change, adopted during an unannounced vote in Parliament on Sunday afternoon, effectively raises the bar for those who oppose the constitution. Given that fewer than 60 percent of registered Iraqis voted in the January elections, the chances that two-thirds would both show up at the polls and vote against the document in three provinces would appear to be close to nil.

"This is a mockery of democracy, a mockery of law," said Adnan al-Janabi, a secular Sunni representative and a member of former Prime Minister Ayad Allawi's party. "Many Sunnis have been telling me they didn't believe in this democratic process, and now I believe they are vindicated."

The rule change could prove a serious embarrassment to American officials in Iraq, who have spent recent weeks struggling to persuade Sunni Arabs to vote in favor of the document and even trying to broker last-minute changes that would make it more palatable to them.

There is some confusion about the origin of the rule change. One member of Iraq's electoral commission said the commission already made a similar ruling last month, while another member denied that. But Ali Dabagh, a moderate Shiite member of Parliament, said there had been no public ruling until Sunday's vote.

Mr. Dabagh also said the United Nations had expressed dissatisfaction today with the rule change, and that the National Assembly would meet on Tuesday to reconsider it.

There were indications from knowledgeable diplomats that the United States, too, was unhappy with the development and hoped that it would be modified.

Other Shiite members of the assembly defended their action. They said that if only people who came to the polls were counted in the referendum, insurgent attacks could frighten so many voters away that the constitution could be rejected on the basis of a small, unrepresentative sample of voters in the most violent provinces.

"You should not violate the rights of the majority," said Maryam Reyes, a member of the Shiite alliance that controls a majority of seats in the national assembly, in support of the measure.

Ms. Reyes said the assembly members had not changed Iraq's election law, but only clarified the meaning of the word "voters" in the relevant passage. The legal passage in question states, "The general referendum will be successful and the draft constitution ratified if a majority of voters in Iraq approve and if two-thirds of voters in three or more governorates do not reject it."

In their vote on Sunday, the Shiite and Kurdish members interpreted the law as follows: the constitution will pass if a majority of ballots are cast for it; it will fail if two-thirds of registered voters in three or more provinces vote against it. In other words, the lawmakers designated two different meanings for the word "voters" in a passage where they appear to mean the same thing.

"I think it's a double standard, and it's unfair," said Mahmoud Othman, a Kurdish assembly member who, as did many other lawmakers, said he had not been present during the vote and only learned of it afterward. "When it's in your favor, you say 'voters.' When it's not in your favor, you say 'eligible voters.' "

In effect, the new interpretation makes not voting a show of support for the constitution and runs against the apparent intent of the law. Iraq's national assembly is empowered to change the transitional law - which was written under the American occupation in 2003 - but only with the approval of two-thirds of its members and of the Presidency Council. Because they regard their action as a mere clarification, the lawmakers did not seek that kind of approval.

Even as the rule change angered Sunnis and some others, there were new signs of tension between the Shiite and Kurdish alliances that dominate Iraq's transitional government. Kurdish leaders threatened to withdraw from the alliance last month, and on Sunday a high-ranking Kurdish official called for the resignation of Iraq's Shiite prime minister, Ibrahim Jaafari. Leaders of the two blocs met tonight to iron out differences on a range of issues, including Kurdish demands for faster Kurdish resettlement in the contested northern city of Kirkuk.

If the Kurds were to follow through on their threat to withdraw, it would be a serious blow to Iraq's government at a critical time. The Shiites have a narrow majority in the national assembly, but if a vote of confidence were called and even a few Shiites were to vote against Mr. Jaafari, the government would collapse, throwing Iraq's public affairs into greater disarray.

Insurgent violence left at least a dozen people dead across Iraq today. In western Baghdad, a roadside bomb exploded on a convoy carrying Iraq's oil minister, Ibrahim Bahr. Three of the minister's guards were killed in the apparent assassination attempt and two were wounded, but Mr. Bahr was unhurt, Interior Ministry officials said. In southern Baghdad's lawless Doura neighborhood, a car bomb exploded, killing two and wounding three. Elsewhere in the capital, three bodies were found with their hands bound and bullets in their heads, the officials said.

In Kirkuk, gunmen opened fire on a police patrol with a belt-fed machine gun, killing two officers and wounding three, police officials said. Two more police officers were killed just south of the city when a roadside bomb exploded on their convoy, the officials said.

South of Baghdad in Hilla, a bomb detonated near a popular restaurant, killing one bystander and injuring three, witnesses said.

The nonprofit Human Rights Watch issued a report today stating that insurgents in Iraq had committed war crimes by making civilians the targets of mass killings, and some of their abuses may constitute crimes against humanity.