"Otherwise, it's just a bunch of empty rhetoric," he said in Iowa.

On Saturday Obama, a Democratic senator, formally threw his hat into the ring as a contender for the 2008 US presidential vote.

The US has close to 140,000 troops in Iraq while Australia has 1,400.

'Significant' contribution

Speaking to reporters on Monday Howard rejected criticism of his comments saying that Australia was making "a very significant and appropriate contribution" to the Iraq war.

But Howard’s Australian critics warned the 'undiplomatic' remarks could harm ties with the US, accusing the prime minister of putting his personal relationship with Bush before Australia's interests.

Opposition leader Kevin Rudd said the prime minister's comments could damage Australia's relations with a future US administration which could be run by a Democrat.

Meanwhile politicians from both the Republican and Democratic parties in the US have condemned Howard’s comments on Obama as interference in domestic politics.

Ron Wyden, a Democratic senator from Oregon, described Howard's comments as "bizarre".

"We'll make out own judgments in this country with respect to elections," he said.

Elsewhere Jon Cornyn, a Republican senator from Texas, said that while Howard may have had a point, it was not his position as the leader of a foreign country to comment.

Elections

The row over Howard's comments comes as a new poll released on Monday showed his support among Australian voters waning ahead of elections expected later this year.

About 65 per cent of more than 1,400 voters in a telephone survey by ACNielsen supported Rudd, the highest popularity rating for an opposition leader in the poll's 35-year history.

Rudd was also the preferred choice for prime minister by 48 per cent of those polled, compared to 43 per cent for Howard.