In Iowa, Democrats have 1 key question. Who is the candidate to beat Trump?

At theQuality Inn & Suites in Ford Madison, Iowa, the state's former governor Tom Vilsackdecidedto let the crowd in on "a little secret."

"You can't governunless you win," he said. "I think all of us have to ask ... who's in thebest positionto win. Joe's in the best position to win."

Joe, of course, is Joe Biden, the former U.S. vice-president. Heseems to be in a tight race with Vermont Sen.Bernie Sandersgoing into Monday night's Iowacaucuses,officially kicking off the U.S. presidential nomination process.

Just moments earlier, Biden had explained to around 150 attendees that he's the oneDemocrat whoDonald Trump fears most.

"It's pretty simple," Biden said. "They don'twant me to be the nominee.I wonderwhy? Because they know if Iam, I'm going tobeat him."

The ability to beat Trumpis,according to Democratic presidential hopeful Andrew Yang, the "number one criteria" for the eventual nominee, and the number one priority for Iowans in choosing their leader tonight.

WATCH: CBC talksto organizers, volunteers in Iowa backing the surging Bernie Sanders, and hopeful Joe Biden

Iowa caucus preview: A real test for Democrats

5 years ago

Duration 2:14

The Iowa caucus has become a pretty reliable indicator for determining who will become the party's nominee.

Tough choices

Despite holding only 41 delegates, the small state, by going first,can play an oversized rolein choosing the nominee. Thewinner herehas often builtmomentum to become the candidate.

Caucuses are gatherings organized by the party where participants debate the candidates' merits and choose their preferred candidate. After the initial vote count is taken, voters backing candidates who earned less than 15 per cent the minimum threshold to earn pledged delegates are free to shift their support to other candidates or go home.

Some Democrats are struggling to choose between the candidate they most support, compared to who they believe is best suited to win.

"The Democrats are torn between do we vote for the guythat's most likely to beat Trumpor do we vote our hearts," said Sherry Martin, a retired locomotive engineer from Montrose, who attendedthe Biden event.

Martin will be supporting Massachusetts Sen.Elizabeth Warren, but her husband Clyde Martinis throwing his support behind Biden, who he believes is the most viable candidate.

"I think Biden is the most electable candidate in the field," hesaid. Vermont Sen. Bernie Sanders, "unfortunately, kind of saddled himself with [the]socialistlabel.

"And Iunderstand he's a socialist Democratbut people out there, they don't know where to file that. They think he's a socialist. And Ithink that did him more harmthan good."

Biden has always been a moderate who has been able to reach across the political aisle, said Iowa resident George Morgan.

Sanders, he said, is "too far left for the United States."

But tell that to a Sanders supporter, and mostly, they scoff.

"The fact is, Trump was unelectablein 2016 ...Noone knows what's electable or not and [Sanders] does have a lot of support behind him," said Eric Ehlers, a hairstylist from Davenport, Iowa.

Ehlers was one of the estimated 3,000 people who flocked into theU.S. Cellular Center arena in downtown Cedar Rapids for a Sanders rally Saturday night, the biggest so far ofhis campaign in the state.

"Ifrigging like Bernie so much," said Heather Stallman, aCedar Rapids dental assistant. "I love him because he's like a normal person. I feellike what you see with him is what you get."

As for the "radical" or "socialist" label, Stallman said: "I don't think that taking care of the peopleas a whole is extreme or radical."

But others too are making the case for electability. At Lincoln High School in Des Moines, his last big rally before the caucus tonight, Democratic candidate Pete Buttigiegtook a not-so-subtle swipe at both Biden and Sanders.

"I know you're goingto see some ads saying that there's only two ways to go. Either you'refor a revolution or you're for the status quo," Buttigiegsaid.

"But the good news for America todayis that we have an historic majority ready not only to rally around what we are againstand get a better presidentbut to come together in the name of what we are for as a country."

Many of his supporters said Buttigiegoffers a fresh face. Heis young, without the Washington baggage of Biden, and more moderate than Sanders.

"I think you have to be in the middle or you're not goingto pull forthepeoplethat you need you can't betoo far to the left or we'll lose," Sheila Burton, an insurance company employee.

John Kerry

There was a report Sunday that an NBC reporter had overheard former secretary of state John Kerry on the phone discussing thepossibility of entering the race out of concern for a Sanders win.

WATCH: The CBC's Adrienne Arsenaultbreaks down the caucus process

How a caucus works

5 years ago

Duration 1:36

The Iowa caucuses are not chess, but they are a bit complicated and they are about choosing a champion. Adrienne Arsenault explains:

In an interview with CBC News shortly after a Biden rally Sunday night,Kerry said that was not true.It would be "impossible" for him to enter, Kerry said. "It's not even realistic."

Kerrywouldn't say if he's concerned about Sanders."Alot of folkshave expressed concernsaboutthe party gettingtoo far in a direction" that would make things more difficult, he said.

He suggested, however, the whole point was moot since Biden will win the nomination and will surprise everyone on Monday.

Speaking to reporters over the weekend, Biden said he thinks, "It's going to be just a tossup here."

"I've said from the beginning I expect to do well. I probably shouldn't tell you that, but I expect to do well," Bidensaid.

Voter turnout

Meanwhile, Sanders said his victory or defeat hinges onone significantfactor voter turnout.

"If there is a low voter turnout, let me be very frank, we're goingto lose. But if there is a high voter turnoutwe're going to win," he told the Cedar Rapids crowd.

He implored his supporters to make certain on Monday night they "have the highestvoterturnout foran Iowa caucus in the history of this state."

That may depend on his campaign's organization, which isparticularly important in a caucus. For a primary or general election, voters may have a 12 -hour window in which to vote, from morning untillate evening.

But for the Iowa caucus,people have to show up Monday night at 7 p.m.

"You've got to hope that the car starts. There's no flat tire. There's not a blizzard. The babysitter shows and so forth," said Drake Universitypoliticalscience professor Dennis Goldford.

"And so it's tougher to get people out for that. So that increases the reliance on organization."

Goldford also said eachcandidate has the same goal: beating expectations.

That could apply to someone like Buttigieg, who is currently polling third, according tothe RealClearPoliticsaggregate poll average.

But for Anne Montgomery, who attended his rally Sunday afternoon, she's confident the former South Bend, Ind.,mayor will do well. Indeed, she said "it's obvious" he's the best candidate to beat Donald Trump.

Obvious at least to Montgomery Buttigieg is her son.