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

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

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

Joe, of course, is Joe Biden, the former U.S. vice-president. He seems to be in a tight race with Vermont Sen. Bernie Sanders going into Monday night's Iowa caucuses, officially kicking off the U.S. presidential nomination process.

Just moments earlier, Biden had explained to around 150 attendees that he's the one Democrat who Donald Trump fears most.

"It's pretty simple," Biden said. "They don't want me to be the nominee. I wonder why? Because they know if I am, I'm going to beat him."

The ability to beat Trump is, 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.

Tough choices

Despite holding only 41 delegates, the small state, by going first, can play an oversized role in choosing the nominee. The winner here has often built momentum to become the candidate. 

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 guy that's most likely to beat Trump or do we vote our hearts," said Sherry Martin, a retired locomotive engineer from Montrose, who attended the Biden event.

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

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

"And I understand he's a socialist Democrat but people out there, they don't know where to file that. They think he's a socialist. And I think that did him more harm than good."

Mark Gollom/CBC

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 unelectable in 2016 ... No one knows what's electable or not and [Sanders] does have a lot of support behind him," said Eric Ehlers, a hairstylist from Davenport, Iowa.

Mark Gollom/CBC

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

"I frigging like Bernie so much," said Heather Stallman, a Cedar Rapids dental assistant. "I love him because he's like a normal person. I fell like 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 people as a whole is extreme or radical."

Mark Gollom/CBC

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 Buttigieg took a not-so-subtle swipe at both Biden and Sanders.

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

"But the good news for America today is that we have an historic majority ready not only to rally around what we are against and get a better president but to come together in the name of what we are for as a country."

Gene Puskar/Associated Press

Many of his supporters said Buttigieg offers a fresh face. He is 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 going to pull for the people that you need you can't be too 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 the possibility of entering the race out of concern for a Sanders win.

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"

Kerry wouldn't say if he's concerned about Sanders. "A lot of folks have expressed concerns about the party getting too 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," Biden said.

Voter turnout 

Meanwhile, Sanders said his victory or defeat hinges on one significant factor — voter turnout.

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

He implored his supporters to make certain on Monday night they "have the highest voter turnout for an Iowa caucus in the history of this state."

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

John Locher/Associated Press

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 University political science professor Dennis Goldford.

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

Goldford also said each candidate has the same goal: beating expectations. 

"The question is not how many delegates did you get," said Goldford, co-author of The Iowa Precinct Caucuses: The Making of a Media Event. "Did you do better than expected or worse than expected? That's the key." 

That could apply to someone like Buttigieg, who is currently polling third, according to the RealClearPolitics aggregate 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.