A week before crucial mid-term elections in the United States, the Iraq war is emerging as the most fiercely contentious issue of the campaign.

In the latest row between Republicans and Democrats, the White House is accusing Senator John Kerry of insulting U.S. soldiers, while the Massachusetts legislator is calling his opponents "right-wing nut jobs".

On Monday, Kerry told students in California that they risked getting "stuck in Iraq" if they didn't study hard and get a good education.

"You know, education, if you make the most of it, you study hard, you do your homework and you make an effort to be smart, you can do well. If you don't, you get stuck in Iraq," Kerry said.

A defiant Kerry said at a news conference Tuesday his comments were a "botched joke" meant to refer to President George W. Bush.

"I apologize to no one for my criticism of the president and of his broken policy," said Kerry.

Earlier in the day, Kerry's office released what it said was his prepared script to the students, which read, "You end up getting us stuck in a war in Iraq. Just ask President Bush."

White House spokesman Tony Snow described Kerry's original remarks as "an absolute insult" to U.S. soldiers.

"Senator Kerry not only owes an apology to those who are serving," he said, "but to those who've given their lives in this."

A bitter campaign

Kerry responded angrily to Snow's comments, as well as the criticisms from conservative broadcasters.

"I'm not going to be lectured by a stuffed-suit White House mouthpiece standing behind a podium or doughy Rush Limbaugh," he said in a statment issued early Tuesday.

"If anyone thinks a veteran would criticize the more than 140,000 heroes serving in Iraq — and not the president who got us stuck there — they're crazy," he added.

Commentators say the tone of the campaign for the midterm elections has rarely been more bitter and intense. Polls show the Democrats set to make substantial gains at most levels, although it's not clear if they'll retake both houses of Congress, which they haven't held since 1994.

The latest opinion surveys indicate a majority of Americans disapprove of the government's handling of the Iraq war, a feeling that Democrats have been appealing to in speeches and advertising.

When he ran against President George W. Bush in the 2004 election, Senator Kerry played up his military service as a decorated Vietnam veteran but the Republican party managed to highlight unsubstantiated allegations about his war record in what became known as the "Swift Boat controversy".

Kerry has admitted that his campaign's slow response to those attacks cost him the election.