Thirty-one US troops died in a helicopter crash in Iraq and five more were killed in insurgent attacks today in the deadliest day for American forces since they invaded the country 22 months ago.
Guerrillas waging a bloody campaign to wreck Sunday's landmark elections also killed 10 Iraqis in a string of bombings and raids today.
As American-led forces pressed their efforts to quell the insurgency and create the conditions for a credible poll, a US Marine transport helicopter went down in the desert of western Iraq. A military official said 31 Marines 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.
President Bush acknowledged the loss of life was "very discouraging" to the American people but appealed for understanding of US goals in the region.
Mounting US deaths have increased public pressure for a clearer exit strategy from Iraq.
Four more US Marines were killed in action in the restive Anbar province of western Iraq, and an American soldier died in a rocket attack north of Baghdad, US officials said.
Calling the upcoming election a "grand moment in Iraqi history", President 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 a coordinated attack, three suicide car bombers hit the town of Riyadh, a Sunni Arab area southwest of the northern city of Kirkuk.
Two cars blew up close to an army post and police station and a third vehicle detonated on a nearby highway.
Four Iraqi policemen, two Iraqi soldiers and three civilians were killed, police said.
A US patrol heading to the scene came under small arms fire and two soldiers were wounded. The previous deadliest day for US forces was March 23rd, 2003, the third day of the war, when 28 US soldiers died mostly in fierce fighting in southern Iraq.
Police in Baquba, a mixed Shi'ite and Sunni town 65 km (40 miles) north of Baghdad, said one Iraqi policeman was killed and at least eight people were wounded when gunmen fired on the local offices of three parties contesting the polls.
Several guerrilla groups in Iraq have declared war on Sunday's elections, vowing to attack polling stations and kill those who vote.
The government plans extraordinary security measures, including closing Baghdad airport and land borders over the election period, extending night curfews in cities and banning cars from roads on election day.