Republican candidates hoping to win the White House in November have just one more full day of campaigning before Tuesday's Iowa caucus — the first major test of the 2012 presidential election.

A poll by the state's most prominent newspaper, the Des Moines Register, makes it hard for experts to predict a winner.

The newspaper puts former Massachusetts governor Mitt Romney on top with 24 percent, closely followed by Texas congressman Ron Paul at 22 percent. Former senator and social conservative Rick Santorum has leapt into third place with 15 percent after spending months languishing at the bottom of voter surveys.

Four other candidates trail — former House Speaker Newt Gingrich, Texas Governor Rick Perry, Congresswoman Michele Bachmann, and former U.S. Ambassador to China, Jon Huntsman. All but Huntsman were at one time near the top of voter surveys.

But the Iowa newspaper says 41 percent of those planning to participate in the caucus are undecided or say they could change their minds before Tuesday, making the outcome far from certain.

While success in the Iowa caucus is not a sure step towards the White House, a strong showing can give a candidate momentum heading into the primary elections. A poor finish in Iowa could also spell the end to a campaign.

The caucus — in which Republicans gather in precinct meetings to pick their favorite — will be followed quickly by primary elections in other states in which candidates will battle to become their party's nominee for the presidential election in November. President Barack Obama is running unopposed for the Democratic party's nomination.