WASHINGTON - Republican presidential candidates have one week to convince Iowa voters that they are the best prepared to defeat President Barack Obama next year.

The Midwestern state holds the country's first nominating contest on Jan. 3 -- statewide precinct caucus meetings. They likely will winnow the seven-person field and shape the coming six-month string of state-by-state primary elections and caucuses leading up to the Republican National Convention that officially names a candidate in August.

Obama is vulnerable because of national dissatisfaction with the wobbly economy that has been extremely slow to pull out of the Great Recession. But the tangle of Republicans vying for the nomination to challenge the president in November -- and fundamental splits over ideology -- have left the party divided.

Republican Party chairman Reince Priebus discounts the chaotic situation, saying the campaign is still in its early stage and calling it "a horse race."

He told ABC television Tuesday that "We'll get there and we'll have a nominee pretty quickly."

National front-runner Mitt Romney, the former Massachusetts governor, is disliked and not trusted by conservative Republicans who have thrown their support behind a series of other candidates whose policy promises are more palatable to them. But each of those candidates, Rep. Michele Bachmann, Texas Gov. Rick Perry, former businessman Herman Cain and now former House of Representatives Speaker Newt Gingrich have all grabbed support and quickly risen in the polls before losing steam under closer scrutiny by voters and the news media.

Many of those expected to take part in the Iowa caucuses remain undecided. And while Romney appears stronger in the state than he had earlier, polls show his biggest rivals are Gingrich and the libertarian-leaning Texas Rep. Ron Paul.

Romney, who finished second in the 2008 Iowa caucuses in his first run for the presidency, released a new television commercial for the state in which he touted his background as a conservative businessman and cited a "moral imperative for America to stop spending more money than we take in. It's killing jobs."

He returns to Iowa on Tuesday after a quick stop in his long-established stronghold of New Hampshire, which holds its first-in-the-nation primary on Jan. 10, exactly one week after the Iowa caucuses.

Romney has a well-funded and well-organized campaign nationally and in Iowa, as well as allies who are spending heavily on television advertisements through an independent organization known as a super PAC, or super Political Action Committee.

He has recently invested more time and resources in Iowa, hoping to eke out a victory in the caucuses at a time when the party's social conservatives have yet to coalesce behind a single candidate.

But Romney hasn't excited many of the party's staunchest conservatives for reasons that include his past support of abortion and gay rights and enactment of a Massachusetts health care plan that's often compared to Obama's overhaul that was pushed through Congress and into law early in his first term.

Bachmann, Perry and Gingrich, each claiming to be the truly conservative alternative to Romney, are launching bus tours Tuesday through Iowa's small towns.

Each campaign has also tried to gauge the level of enthusiasm for Paul. The libertarian favourite has built a strong organization in Iowa and recent polls suggest he is peaking, a rise that has him tied with or even ahead of Romney -- and drawing more scrutiny for his views -- particularly newsletters that appeared under his name two decades ago with comments about "race war" and deriding the influence of the pro-Israel lobby.

Paul was set to return Wednesday for a late push ahead of the New Year's holiday in which he planned to meet with supporters he has kept in touch with since his unsuccessful run in 2008.

"There's really three primaries going on here," former Pennsylvania Sen. Rick Santorum told reporters in Adel, Iowa, where he went hunting for pheasant and quail. "There's the libertarian primary, which Ron Paul is going to win. Then you've got the moderate primary, which Gingrich and Romney are scrumming for. And you've got three folks who are running as strong conservatives."

Santorum, who has invested more time in Iowa than any other contender, included himself, Bachmann and Perry in that conservative camp.

Former Utah Gov. Jon Huntsman signalled early on he would not compete in Iowa and instead plans to start his campaign in New Hampshire.