BAGHDAD -- Iraq's torturous effort to hold parliamentary elections on schedule in January collapsed on Monday, raising the prospect of a political and constitutional crisis next year as the United States begins withdrawing the majority of its combat troops.

After two days of divisive sessions and failed talks, Iraq's Parliament disregarded a veto by one of the country's vice presidents and approved new amendments that the vice president promptly indicated he would veto as well.

The moves deepened a crisis that had fleetingly seemed resolved after months of wrangling over how to organize the national election, widely seen as a barometer of Iraq's progress toward democracy.

The failure to agree on even the terms of the national election has inflamed ethnic and sectarian tensions that had waned somewhat in the last year or so.

"Now we have only bad choices," Ahlam Asad, a Kurdish member of parliament who supported the new law, said as lawmakers filed out of their session, which was held, as usual, behind closed doors.

The Parliament, known as the Council of Representatives, does not appear to have the necessary three-fifths majority to override a new veto, making it impossible, several senior lawmakers said, to hold the vote in January as required under the country's Constitution. The Obama administration and the American commander in Iraq, Gen. Raymond T. Odierno, have long planned the withdrawal of American forces around the expectation that the election would take place in January.

There are now roughly 120,000 troops in the country, and a significant withdrawal, known in the Pentagon as "the waterfall," is scheduled to begin in the spring. Under President Obama's policy, fewer than 50,000 troops are to remain in Iraq after August of next year.

Last week, after Mr. Hashimi's initial veto, General Odierno said he would "not have to make any decisions until about late spring." Even so, it appears certain that the election will not be held until at least the beginning of the major withdrawals.

In Washington, Secretary of State Hillary Clinton expressed hope that the election would be held though it "might slip by some period of time until this is worked out."

"We have every reason to believe that elections will be held, which will be another milestone on the journey that Iraqis are taking toward full and comprehensive democracy," she said.

Parliament voted on the law Monday despite frantic efforts by American and United Nations diplomats at Parliament to broker a compromise between the vice president, Tariq al-Hashimi, and lawmakers who accused him of trying to bolster the chances of Sunnis by widening a quota for Iraqis living abroad. But the Parliament did not override Mr. Hashimi's veto. Instead the Shiite and Kurdish blocs amended the election law -- 133-17 -- after Sunni and secular parties walked out of the session in protest. Of the Parliament's 275 members, 125 did not vote or did not attend -- most of them Sunnis, though not all.

Prime Minister Nuri Kamal al-Maliki and his bloc supported the new law, having sharply criticized Mr. Hashimi's veto. A spokesman, Ali al-Mousawi, said that Mr. Maliki favored holding an election as soon as possible.

The three-member Presidency Council, which includes Mr. Hashimi, President Jalal Talabani, and a second vice-president, Adel Abdul Mahdi, now has 10 days to approve or veto the new law.

"The government will keep working as it is right now, and we will not postpone the elections too much," Mr. Mousawi said. "The Council of Representatives has reached a new formula. If there is a veto, then it is going to be different matter."

The Parliament's constitutional mandate expires on March 15, and an election is supposed to happen 45 days before that, Jan. 30.

Iraq's Constitution, written with American and other foreign assistance in 2005, allows a one-month extension. In a country with little democratic or legal precedence, the question of what happens if a new Parliament has still not yet been elected at that point remains anything but clear.

As a result a sense of crisis loomed over Monday's session, with lawmakers on each side accusing the other of violating the Constitution to gain advantage in the election.

"It's a conflict of wills," Osama al-Nujaifi, a Sunni member of Parliament from Nineveh, said angrily after Monday's vote, complaining that the newest version of the election law would disenfranchise Sunni voters.

Many officials afterward warned that the election could be delayed until March or even later. With an important Islamic holiday beginning this weekend, it is unlikely that any resolution of the dispute can begin until next month, after the Presidency Council reviews the new law. The Parliament does not have another session scheduled until Dec. 8, a spokesman for its speaker said.

Mr. Hashimi vetoed the first election law after objecting to a clause that reserved 5 percent of seats in the new Parliament for small religious and ethnic minorities and Iraqi refugees abroad, many of whom are believed to be Sunnis opposed to the government. He said the quota should be raised to 15 percent.

In response to the veto, Shiite and Kurdish parties rewrote the rules for calculating the voter rolls and thus the distribution of seats -- to their own benefit, according to Sunnis. The new law still reserves seats for minority groups, but counts Iraqis abroad in the provinces they fled from rather than as a separate quota.

Mr. Nujaifi said under the new law, Sunnis in Nineveh would lose as many as eight seats, assuming voters cast their ballots on ethnic lines. Nineveh and Kirkuk in the north include disputed territories claimed by Arabs and Kurds.

Some Sunnis suggested the veto had backfired, leaving Sunnis living abroad in a worse position. "Let Mr. Hashimi take his veto back and release us from this impasse," Hassan Dekan, a Sunni member, said.

Mr. Hashimi sent a letter to Parliament on Monday that appeared open to a compromise, but it was not clear that the Shiite and Kurdish majority even discussed it.

"Unfortunately the first law was unjust and irresponsible to the displaced people and those abroad," Mr. Hashimi's spokesman, Abdul Ellah Kadhim, said after the vote, "while now it is unjust and irresponsible to the people abroad and inside Iraq as well."

The threat of violence, often with a political hue, remains ever-present in Iraq. And shortly after the vote, gunmen in a passing sedan fired at a motorcade belonging to the former prime minister, Ayad Allawi, a Shiite who recently aligned with Mr. Hashimi. Mr. Allawi was not in the motorcade at the time; two of his guards were wounded, according to the Ministry of the Interior.

Riyadh Mohammed, Saad al-Izzy, Duraid Adnan and Mohammed Hussein contributed reporting.

By Steven Lee Myers
New York Times