Senator Bernie Sanders, a self-styled socialist, is ahead of the moderate former Vice President Joe Biden by 4 per cent in the voter polls of Iowa Democrats, according to polls aggregator RealClear Politics.
But his lead is at risk from the second preference vote built into the Iowa voting system.
In the US election system, party members elect their party candidates rather than giving party bosses the sole authority to nominate the presidential challengers.
There are two ways this is done - through secret ballot in the system of primaries, and open voting in the caucus system.
Iowa is one of the few states that follow the caucus system.
The registered party members will gather at 7 p.m. on Monday in designated places and instead of casting their ballots secretly, they will go and stand in areas designated for each candidate and another for the undecided.
After an initial count of the voters, those candidates getting less than 15 per cent support will be eliminated and voters standing in the spots designated for them can move to those of their second preference.
At this time, voters can also go to supporters of other candidates and ask them to switch sides.
The voters are then locked in place for a second count and the results are then announced.
The caucus system is criticised for lack of secrecy, which can make people vote in a certain way against their judgment because of peer pressure and because in elections for local offices the officials can identify who opposed them.
The fixed time can also depress the voter turnout.
On the other hand, it has the advantage of allowing second preference votes instead of allowing the votes for low performing candidates to go to waste. And the results can be instantly tallied within the hour allotted for the caucuses.
As in the US presidential system, the caucuses and the primaries are in theory indirect elections.
Iowa voters like all others will, in reality, be selecting delegates pledged to support the candidate of their choice.
Each candidate will be allocated delegates based on their votes through aAformula.
When they go to the party conventions, the delegates will elect the presidential candidate.
This is similar to the national presidential election where the voters actually elect members of the electoral college who then elects the president.
Therefore, it is possible for the candidate who gets the most votes to lose the presidency because the number of rival's members of the electoral college is higher.
In 2016, Trump won with 2.86 million fewer votes than his rival Hillary Clinton because he had 77 more electors in the Electoral College.
In Iowa there are about 657,000 registered Democrats who will be eligible to go participate in the caucuses held on Monday at 1,600 precincts, which are usually public buildings like schools and libraries, but can also be churches or private houses.
he next stop in the Democratic presidential race is New Hampshire, which will hold a primary on February 11.
A clearer picture of the field will emerge on March 3, known as Super Tuesday, when 14 states, including California, will be selecting the Democratic candidate.
Two notable late entrants to the Democratic race, billionaire media and financial information mogul Michael Bloomberg and former Massachusetts Governor Deval Patrick, who have skipped Iowa and the three other early states will be on the ballots on Super Tuesday.
While the Iowa polls can give an idea of who the front-runners are, by no means it establishes the final results.
In 2008, former President Barack Obama had established his electability by winning Iowa and showing himself as a strong and viable candidate.
But Trump had lost in Iowa to Senator Ted Cruz in 2016, and former Democrat President Bill Clinton to Senator Tom Harkin in 1992, but both finally were the winners of their party nominations.
The primaries and caucuses will continue until June 6, when the US Virgin Islands territory will hold its caucus.
Democrats living abroad will elect 13 delegates with one vote each pledged to candidates to participate in the convention and eight other delegates with half-a-vote each.
The Democratic Party will hold its convention July 13-16 in Milwaukee to crown its candidate ahead of the presidential election on November 3.
If there is no clear winner by the convention, candidates can negotiate on policy platform items to trade their delegates and the party bosses can have a more direct role in helping determine the candidates.
Although the RealClear Politics aggregation of Iowa voter polls showed Sanders ahead of Biden by 4 per cent and the Emerson poll released on Sunday gave the Vermont Senator a 7-point lead, the use of second preference does not guarantee him a win as he can be outvoted if the trailing candidates' supporters switched to Biden or someone else.
In the polls aggregation, South Bend, Indiana, Mayor Pete Buttigieg is in the third place, followed by Senator Elizabeth Warren.
(Arul Louis can be contacted at arul.l@.in and followed on Twitter @arulouis)
( With inputs from IANS )