MASON CITY, Iowa/DAVENPORT, Iowa (Reuters) - Kristen Marttila braved sub-freezing temperatures on Saturday to knock on doors in Mason City, Iowa, trying to convince voters to cast their lot with Democratic presidential candidate Elizabeth Warren in the state’s nominating contest on Feb. 3.

Time after time, Marttila said, she heard the same message: People loved the senator from Massachusetts but were concerned her liberal stances would not draw enough broad support to defeat U.S. President Donald Trump in November.

“They really like you – they might even like you the best,” Marttila, a 39-year-old lawyer from Minneapolis, told Warren at a campaign event later that day. “But they are really scared to vote for who they like the best, because they’re worried that not enough people feel the same.”

With only three weeks until the crucial first-in-the-nation contest in Iowa, the sprawling field of 13 Democratic candidates is still struggling to convince an electorate desperate to oust Trump from office which one of them is best positioned to do so.

The race remains fluid, with the top four contenders – U.S. Senator Bernie Sanders, Warren, former South Bend, Indiana, Mayor Pete Buttigieg and former Vice President Joe Biden – bunched between 15 and 20 percentage points in the latest Des Moines Register poll of Iowa Democrats. [nL1N29F1U3]

In interviews this weekend with dozens of Iowa voters, many said they were grappling with the head-or-heart dilemma Marttila outlined – whether to support the candidate who most appeals to them, or the one they imagine will appeal to everyone else.

“I still think she’s going to scare off a lot of voters,” said Michael Marth, 68, a retired farmer and undecided voter who attended Warren’s town hall in Mason City on Saturday.

His wife, Leah, was even more blunt, saying she probably likes Warren the most but will likely end up in Biden’s camp.

“To get rid of Trump,” she explained. “If it was any other year, I’d be on her side.”

Slideshow ( 6 images )

The vaguely defined notion of “electability” has dominated the Democratic contest, given the deep distaste for Trump among the party’s faithful. The latest Reuters/Ipsos poll this month showed twice as many Democrats as in the fall of 2015 say their top concern is whether a candidate can win the general election.

Some voters argued electability can be shorthand for discrimination against women or minority candidates.

“I think a lot of the ‘unelectability’ thing is just misogyny,” said Alex Farrell, a 33-year-old hospital worker who supports Buttigieg but has Warren as a second choice.

The issue is particularly acute for Warren and Sanders, whose backing for ambitious policies like Medicare for All has unnerved moderate Democrats worried that Trump will caricature them as radical socialists.

INDEPENDENTS AND UNHAPPY REPUBLICANS

Fran Henderson, a retired pharmacist who watched Sanders speak on Saturday in Newton, Iowa, said she liked Sanders’ record on issues like the Iraq war but feared policies like Medicare for All and tuition-free college would scare off voters who oppose higher taxes.

“I don’t think he’s going to bring the independents that we need to win,” said Henderson, who supports moderate U.S. Senator Amy Klobuchar. “We need the independents and we need the unhappy Republicans.”

Biden and Buttigieg, the two leading moderates in the field, have questioned whether more liberal candidates like Sanders and Warren will cost the party in November. Biden in particular has asserted that he can win back the traditionally Democratic white, working-class voters who defected to Trump’s side in 2016.

Slideshow ( 6 images )

In response, Sanders and Warren have emphasized their economic populism, calling for structural changes to address growing income inequality and focusing on the Trump tax cuts that largely benefited the wealthy.

“The way you defeat Trump is talk to working people in this country who are so exasperated, so tired of working longer hours for lower wages, so tired of working for 10, 11, 12 bucks an hour and not being able to afford to maintain their family with a dignity that family deserves,” Sanders said on Saturday in Davenport, where campaign workers handed out “Bernie beats Trump” placards.

Plenty of Warren and Sanders supporters have dismissed concerns about the candidates’ electability, saying their bold agendas are more likely to inspire young voters and mobilize the party’s base to turn out in November.

Warren’s answer to Marttila’s concerns was one she has made countless times on the campaign trail: Democrats can only win by standing up for “big ideas,” not incremental changes.

“A lot of people just want to beat Donald Trump,” Warren said. “But here’s the thing: fear doesn’t win. Courage and vision win.”