BAGHDAD, January 26, 2005 (Reuters) - Thirty-one U.S. troops died in a helicopter crash in Iraq and six more were killed in insurgent attacks on the deadliest day for American forces since they invaded the country 22 months ago.

Rebels waging a campaign to wreck Sunday's elections also killed 25 Iraqis in a string of suicide bombings and raids on Wednesday, some of them aimed at parties contesting the polls.

President Bush urged Iraqis to "defy the terrorists" and vote in large numbers.

Several guerrilla groups, including that led by al Qaeda's leader in Iraq, Abu Musab al-Zarqawi, have declared war on the election, vowing to attack polling stations and kill those who vote.

Defense Secretary Donald Rumsfeld said he expected violence to remain at high levels after the election because insurgents might exploit the uncertainty while results are confirmed and the government assembled.

"You're looking at, I would guess, well into March, possibly into April, for these things to sort themselves," Rumsfeld said in Washington.

While U.S.-led forces worked to quell the raging insurgency and create conditions for a credible election, a U.S. transport helicopter went down in the desert of western Iraq. Lt. Gen. John Sattler said 30 Marines and a sailor were killed.

The cause of the crash near the town of Rutba, close to Iraq's border with Jordan, was not immediately known, but there were signs that bad weather may have been a factor.

Bush acknowledged the loss of life was "very discouraging" to the American people but appealed for understanding of U.S. goals in the region. Mounting U.S. deaths have increased public pressure for a clearer exit strategy from Iraq.

Four U.S. Marines were killed in action in western Iraq's Anbar province and an American soldier died in a rocket attack north of Baghdad, U.S. officials said. Another soldier was killed by a roadside bomb in the capital.

The previous deadliest day for U.S. forces was March 23, 2003, the third day of the war, when 28 U.S. soldiers died, mostly in fierce fighting in southern Iraq.

SOWING FEAR

The latest surge of insurgent attacks appeared aimed at sowing fear even as the U.S.-backed interim government vowed stringent measures to safeguard the election, Iraq's first since the fall of Saddam Hussein in April 2003.

Calling the election a "grand moment in Iraqi history," Bush said at a news conference: "I urge all people to vote. I urge people to defy the terrorists... they have no clear view of a better future. They're afraid of a free society."

In the bloodiest attack on Wednesday, a truck bomb exploded near the offices of the Kurdistan Democratic Party in the northwestern Iraqi town of Sinjar, killing 15 people and wounding 30, the local mayor, Dakheel Kassim Hassoun, said.

Zarqawi's Islamist group said it was responsible.

Earlier on Wednesday, three suicide car bombers hit Riyadh, a Sunni Arab town near the northern city of Kirkuk. Four Iraqi policemen, two Iraqi soldiers and three civilians were killed.

Police in the town of Baquba, north of Baghdad, said one policeman was killed and eight other people wounded when gunmen fired on the offices of three parties running in the elections.

Sunni insurgents have repeatedly attacked the country's fledgling security forces in the countdown to the election, accusing them of collaborating with U.S.-led occupiers.

WAR DECLARED ON ELECTION

In the northern city of Mosul, a video filmed by insurgents showed three Iraqi men who had apparently been taken hostage and who said they worked for Iraq's electoral commission.

The government plans extraordinary security steps over the election, closing Baghdad airport and Iraq's borders, extending night curfews and banning cars from roads on voting day. Zarqawi, a Jordanian with a $25-million bounty on him, says the election is a plot by Washington and Iraqi Shi'ite allies against Sunni Arabs, who fear being marginalized.

Iraq's Shi'ites strongly support the elections. A list of candidates dominated by Shi'ite Islamists and drawn up with the guidance of revered cleric Ayatollah Ali al-Sistani is expected to win the most votes, cementing their newfound political power.

Many Sunni parties are boycotting the polls, saying the insurgency raging in their central heartlands will prevent supporters from voting and this will skew the results.

A low Sunni turnout could undermine the credibility of the election and add to fears of sectarian strife.
(Additional reporting by Aref Mohammed in Kirkuk, Shamal Aqrawi in Arbil and Luke Baker, Andrew Marshall and Gideon Long in Baghdad and Vicki Allen in Washington)