After months of campaigning and millions of dollars spent, Republican and Democratic U.S. presidential candidates are set to face their first real test in Iowa on Thursday night.

In the official kickoff to the November 2008 presidential election, the state's eligible voters from both parties will take part in caucuses to choose who they believe should be the next commander-in-chief.

Democratic presidential hopeful Senator Hillary Rodham Clinton speaks during a campaign stop in Cedar Rapids, Iowa, Wednesday.
(LM Otero/Associated Press)

Candidates on Thursday made last-ditch pleas for support from Iowa voters, appearing at rallies across the state and granting interviews on morning television talk shows.

When the caucuses start at 7 p.m. CT, voters will indicate their preferred candidates and choose delegates to attend party conventions later this year.

Polls have suggested a three-way race among Democrats and a two-way contest between the Republicans.  

On the Democrat side, Senator Hillary Rodham Clinton, considered the front-runner when the campaign began, Senator Barack Obama and former senator John Edwards all appear to be running neck and neck in the state.

"This country is ready for a leader who will bring us together," Obama said in a two-minute commercial televised at the dinner hour.

Clinton's campaign has focused on her experience and time in the White House during her husband's two-term presidency.

"If you stand with me for one night, I will stand up for you every day as your president," she said on Wednesday. "I'll work my heart out to bring the country we love the new beginning it needs and I will be ready to start on Day 1."

But with Obama climbing in the polls, Clinton's campaign has rapped the junior Illinois senator for what they say is his inexperience.

Obama, who has campaigned on a theme of change and bipartisan co-operation, has rebuffed Clinton's charges and questioned whether being the spouse of a president makes one qualified for the role.

A member of the audience listens as Democratic presidential hopeful Senator Barack Obama speaks at a campaign stop Wednesday.
(M. Spencer Green/Associated Press)

Edwards, the party's 2004 vice-presidential nominee, has portrayed himself as the defender of the little guy. He has cultivated the union vote, the rural vote and campaigned relentlessly in Iowa hoping to build up enough steam to give him staying power in the primaries to follow.

David Yepsen of the Des Moines Register, considered the dean of political reporters in Iowa, believes Iowa is very important for Democrats.

"Hillary Clinton wins this, she could re-ignite her campaign. If she loses, stumbles, then I think Barack Obama is in a position to run the table and certainly John Edwards lives to fight another day, in another state."

In the Republican field, Baptist minister and former Arkansas governor Mike Huckabee has come from behind to slightly lead the race in Iowa and become a national contender. He faces a tough fight against former Massachusetts governor Mitt Romney, who has outspent Huckabee by a ratio of 20 to 1. 

"You just don't know what is going to happen," Romney said Wednesday, unwilling to forecast success over Huckabee.

Former Sen. Fred Thompson of Tennessee, who was a late entry to the race, was hoping for a third-place finish to rescue what some have labelled a faltering candidacy.

Rudy Giuliani, the former New York City mayor considered the national front-runner but who has seen his support slip, has effectively bypassed Iowa to concentrate his resources on big states like Florida.

Arizona Senator John McCain, who's moving up in countrywide polls, has also moved beyond Iowa, spending most of Wednesday in New Hampshire, which holds a primary on Jan. 8.

Huckabee's conservative views about abortion and gay marriage have rallied the religious right and helped him shoot up in the polls. Some see Romney as a flip-flopper because he changed his political stance on abortion, having once supported abortion rights.

Republican presidential hopeful and former Arkansas governor Mike Huckabee greets audience members at a campaign stop in Mason City, Iowa.
(Paul Sancya/Associated Press)

"You can look at my record and find out that all the way back, as far as you can find me saying or doing anything, I believe the sanctity of human life is a key, critical cornerstone issue for the future of our country," Huckabee said during his Wednesday appearance on the Tonight Show with Jay Leno.

Romney, a successful businessman, has narrowed the gap with negative ads about Huckabee's record of being more forgiving toward illegal immigrants, granting clemency to criminals and raising taxes.

Huckabee, who has accused Romney of running a dishonest campaign, was set to fire back. He had called a news conference Monday to say he was going negative in Iowa and to play an ad he was to air. Instead, he said he had a last-minute change of heart and would not air it, but he played if for the media anyway, sparking laughter among journalists.

Romney's Mormon faith has also become an issue during the campaign, causing concern among some Christian conservatives who view it as a cult. Huckabee himself raised questions about some of the religion's beliefs in an interview, but later apologized.

The issue prompted Romney to deliver a speech in December, asking Americans not to reject his presidential bid because of his Mormon faith.

Whoever wins in Iowa, a victory would likely give that candidate a boost in New Hampshire, where independents can vote in either primary. But it should be clear by Super Tuesday on Feb. 5, when more than 20 states weigh in, who the nominees will be.

With files from the Associated Press