On the Democratic Party side, the gap between former secretary of state Hillary Clinton and Senator Bernie Sanders is even narrower, with Mrs Clinton ahead by just 3 percentage points.

The poll has predicted the correct winner in eight out of the last nine races it called.

The razor-thin margins contributed to the all-out push by candidates over the weekend. On Sunday, most held at least four events, each one in different parts of the state.

Many called in their star power for the finale. Mr Cruz had talk show host Glenn Beck and Duck Dynasty star Phil Robertson pitch for him, while Mr Sanders' campaign trotted out the band Foster The People and actor Josh Hutcherson, who is best known for playing the role of Peeta Mellark in the Hunger Games movies. Mrs Clinton, in turn, counted on her family, daughter Chelsea and husband Bill Clinton, the president from 1993 to 2001.

On the Democratic Party side, both Mr Sanders and Mrs Clinton used their rallies to underscore what they saw as their key strengths, with only veiled criticism of each other.

Mr Sanders continued to push his message about narrowing income inequality and going after Wall Street, while Mrs Clinton stressed her qualifications and proven ability to get things done.

Former president Bill Clinton, without naming Mr Sanders, said pragmatism was more important for a president than just "having good positions".

Though it was brought up by some Republican candidates, the revelation last Friday that several of Mrs Clinton's unsecured e-mail when she was secretary of state may have contained top secret information did not make a splash on the campaign trail.

On the Republican side, much of the attention on the weekend was focused on Mr Cruz, who came under fire for a mailer he sent out to voters that was done in the style of an official government notice, warning them that not voting in the Iowa Caucus was a violation. There is no such violation in Iowa.

Mr Trump jumped on it quickly, tweeting: "The Cruz campaign issued a dishonest and deceptive get out the vote ad calling voters 'in violation'. They are now under investigation. Bad!"

Mr Cruz's mailer, though, did underscore the importance of turnout at the caucuses. Mr Trump and Mr Sanders have significant leads among first-time caucus attendees and their success will depend heavily on whether or not they can get those people to turn up.

A snowstorm expected to set in late on Monday night added to the uncertainty of the outcome, with bad weather likely to depress turnout.

At this stage of the race, it was clear that most people in Iowa have already made their minds up about whom they were going to vote for.

Mr Matt Lorimor, 28, an information security engineer, said he believed Mr Sanders would win if young people would come out to vote in greater numbers.

"Everybody around my age that I know supports him. It's just a matter of whether they will turn up."

Mr Tom Lewis, 51, unemployed, in turn said he is leaning heavily towards voting for Mr Cruz. "Mr Trump says the same thing over and over again and I don't believe him," he said.

Mr David Yepsen, director of the Paul Simon Public Policy Institute at Southern Illinois University, said one big issue that will play at the caucuses is whether or not candidates can win the general election.

"On the Republican side, that's a big argument. Is Donald Trump electable? Is Ted Cruz too conservative? And the same is true on the Democratic side, where you have the argument that Hillary Clinton is making that Bernie Sanders is just simply too left of centre.

"You'll hear that argument at your caucus on caucus night, whichever one of those you go to, that activists will be arguing among themselves over who can win and who won't."

• Additional reporting by Melissa Sim