Iraq was thrown into a fresh political crisis Wednesday after the vice president vetoed a newly passed election law, delaying the Jan. 18 vote, setting off fresh sectarian wrangling and possibly complicating plans to withdraw U.S. troops.

In a move that caught U.S. officials by surprise, Vice President Tariq al-Hashemi , a Sunni Muslim, said he had vetoed the new election law, saying the measure doesn't provide adequate representation for millions of displaced Iraqis, most of whom are thought to be Sunnis.

Afterward, the chief executive of Iraq's United Nations-supported electoral commission said that the elections would have to be put off.

The veto touched off a political explosion. Iraq's prime minister, Nuri Kamal al-Maliki, condemned it as constitutionally questionable, while President Jalal Talabani, a Kurd, warned that delaying the election risked creating a constitutional vacuum during which the Iraqi government would lose its legitimacy. Other Kurdish leaders threatened to boycott the elections altogether.

"Parliament could amend this law in a day," Al-Hashemi said. "We have no time to lose."

But with Kurdish leaders also objecting to provisions of the law, a much more protracted debate in Parliament is likely.

The United States has consistently pushed national elections both as an important step toward reconciling still-yawning Iraqi political divisions and as a prerequisite for withdrawing U.S. troops.

Gen. Raymond Odierno, the commander of U.S. forces in Iraq, said he had until about May 1 to decide whether to ask President Obama to alter the timeline to reduce to 50,000 U.S. troops -- from about 120,000 -- in Iraq by Aug. 31. All U.S. forces are due to be out of Iraq by the end of 2011. Odierno said there were now 115,000 U.S. troops in Iraq, a slight reduction from recent levels.