PELLA, Iowa (AFP) - The US presidential marathon entered the home stretch Thursday ahead of the first official contest of the race in Iowa in exactly one week's time.

Nominating Democratic and Republican Party caucuses, set for January 3 in this Midwestern farm state, formally kick off this year's election contest that will culminate with a November 4 national vote.

US Senator Hillary Clinton entered the phase with an urgent warning that only she can solve America's most pressing problems.

The former first lady is consciously raising the stakes and suggesting that choosing one of her rivals over her may be a huge risk.

"It is time to pick a president," Clinton told voters in Iowa on Wednesday.

As the caucuses loom, Clinton is developing the theme that helped her coast through much of the year with a gaping lead in key opinion polls, hoping to quell a serious of setbacks and missteps that plagued her earlier in December.

As she stood in front of a huge American flag Wednesday, in a high school 40 miles, (60 kilometers) from Iowa state capital Des Moines, Clinton painted a dire picture of an America under siege at home and abroad.

She said the next president had a war in Iraq to end, to offer healthcare to 47 million uninsured Americans, and to deal with an economy she said was tipping into a deep crisis.

"I am not coming to you with promises about what I think I can do or hopes that together we can achieve some of these ends .... but with a track record and an understanding of how difficult the process is," Clinton said.

Illinois Senator Barack Obama, locked in a gruelling fight for the Democratic nomination with Clinton, is also campaigning hard in Iowa. In his first post-Christmas swing, he took a veiled swipe at the former first lady, accusing her of running an "old textbook Washington campaign".

"You know, telling the American people what they want to hear instead of what they need to hear just won't do," Obama said.

On the Republican side, Mitt Romney launched a fierce new attack on rising rival Senator John McCain, who is threatening his lead in New Hampshire, accusing him of supporting amnesty for illegal immigrants.

Tiny northeastern New Hampshire holds its bellwether primaries on January 8.

"I know something about tailspins, and it's pretty clear Mitt Romney is in one," McCain, a former navy aviator, responded in a statement.

"It's disappointing that he would launch desperate, flailing and false attacks in an attempt to maintain relevance."

Counted out by many only one month ago, McCain has pulled into second place in New Hampshire and third in Iowa in the most recent polls.

Four of the Republican candidates -- Mike Huckabee, Rudy Giuliani, Romney and McCain -- remain so close in the earliest states to vote that none can be counted out.

Breakout contender Huckabee, a Baptist minister and former Arkansas governor, has a convincing lead in Iowa, while former Massachusetts governor Romney is ahead in New Hampshire.

Huckabee on Wednesday ventured out onto a frozen Iowa field to shoot pheasant, in a photo-op which captured the day's headlines in the Republican race, and also likely endeared him to the powerful gun lobby and rural voters.

Giuliani, the national frontrunner among Republicans, had taken the high-risk strategy of largely bypassing Iowa and New Hampshire to focus on Florida, which votes on January 29, and then other big states such as California and New York which vote on February 5.

But with his national poll lead slipping, the former New York mayor was to join the rest of the field in Iowa after Christmas, promising to go all out after a brief hospital stay last week for "flu-like symptoms".

"The Republicans are starting to see you really can't skip Iowa," Chuck Laudner, who head the state's Republican party, told the Los Angeles Times on Wednesday.

The early contests tend to give states where they are held a disproportionate role in the selection process, and often bring surprises, sometimes knocking a perceived frontrunner off the field.

The Democratic and Republican presidential candidates will be formally nominated at their respective party conventions in late August and early September, but the nominee is generally known long before then.

AFP, December 26, 2007