DES MOINES, Iowa - Republican Mitt Romney is looking with optimism at Iowa, the state that rejected him four years earlier but now appears at least open to the possibility that he could be his party's presidential nominee.

As Iowa closes in on its caucuses Tuesday, which will mark the country's first voting in the state-by-state nominating contest, Romney is suddenly making a public play to win the state he largely kept at arm's length since his stinging second-place finish in 2008.

Whoever wins the Republican nomination will run next November against President Barack Obama, who is vulnerable as he seeks a second term, weighed down with voter dissatisfaction over his handling of the economy and the stagnant recovery from the recession.

But Romney is viewed by many as too moderate, and libertarian Texas Rep. Ron Paul has emerged as his chief rival in Iowa despite some concerns over his isolationist views on foreign policy.

Social conservatives dissatisfied with Romney remain splintered among a handful of candidates that include Texas Gov. Rick Perry, Minnesota Rep. Michele Bachmann and former Pennsylvania Sen. Rick Santorum.

Romney planned to begin his Friday in West Des Moines with New Jersey Gov. Chris Christie, a larger-than-life figure whom conservatives courted to join the race before he endorsed Romney's presidential quest.

Romney's declared rivals, meanwhile, were working in overdrive to emerge as his chief opponent. But none went directly after the former Massachusetts governor. Instead, they kept their focus on each other as all hoped a strong showing here could yield momentum heading into the next contests in New Hampshire and South Carolina.

"Don't settle for what's not good enough to save the country," the newly ascendant Santorum told Iowans on Thursday in the town of Coralville. He urged voters to put conservative principles above everything else and suggested that his rivals, and specifically Ron Paul, lacked them.

The manoeuvring underscored the fluid -- if not convoluted -- state of the Republican presidential race as Tuesday's caucuses loom while cultural conservatives and evangelical Republicans, who make up the base of the electorate here, continue to be divided. That lack of unity could pave the way for someone who is seen as less consistently conservative.

Five days out, public and private polling show Romney and Paul in strong contention to win the caucuses, with coalitions of support cobbled together from across the Republican political spectrum and their get-out-the-vote operations -- beefed up from their failed 2008 bids -- at the ready. They're the only two with the money and the organizations necessary to ensure big turnouts on Tuesday.

The three others -- Santorum, Perry and former House of Representatives Speaker Newt Gingrich -- will have to rely largely on momentum to carry supporters to precinct caucuses. Each was working to convince fickle conservatives that he alone would satisfy those who yearn for a nominee who would adhere strictly to the Republican Party orthodoxy.

After the Iowa caucuses, the Republican hopefuls will have to step up campaigning in New Hampshire, which holds the nation's first primary vote on Jan. 10.