VINTON, Iowa — As Elizabeth Warren rises in the polls, Democratic voters are concerned the Massachusetts senator isn't electable enough to defeat President Trump in 2020.

“We saw Warren up close, and you could see her eyes, and she was intense, there's no question about that. I think she takes off too big a bite as a candidate,” said Jim Mikulanec, a 70-year-old retired sales manager who saw Warren in Indianola, Iowa.

Warren’s refusal to say whether middle-class taxes would go up in order to fund her proposed “Medicare for all” healthcare plan is also causing jitters.

“She lacked what I call critical substance on healthcare,” Mikulanec said. “Where did they get the money? How do they justify it? How does it come about?”

The questions come as Warren can boast of a place in the top-tier of the 16-person Democratic primary field. She still trails former Vice President Joe Biden in the RealClearPolitics average of national polls, 28.8% to 24.2%, but that represents a steady climb from spring and early summer when she was in the single digits.

Erin Monaghan, a 54-year-old nonprofit director who attended an event in Vinton, Iowa, for California Sen. Kamala Harris, recognized that the risk in nominating Warren, 70, is that she might not capture older age groups. “If the younger voters turnout, she'll be fine. If it's a generation and older, they're probably not gonna vote for her,” Monaghan said.

[ Also read: With a spring in her step, Warren, 70, breezes past scrutiny of her age]

Democrats are anxious about whether Warren, who has positioned herself on the left end of the Democratic primary field, is up to the task up beating Trump in swing states, Democratic strategist Jeff Hewitt told the Washington Examiner.

Those who drive liberal politics in the Democratic Party mostly reside in states that will likely support the Democratic presidential candidate no matter what.

“If you're trying to win Ohio, Michigan, Pennsylvania, Indiana, Wisconsin, states like that, we don’t have progressive statewide elected officials in those states,” Hewitt said, because “the electorate and the Democratic Party is not as progressive.”

Paul Henderson, a Bay Area political analyst who previously worked in the San Francisco mayor’s office and district attorney's office under Kamala Harris, told the Washington Examiner that electability is “always the ultimate issue” for the Democratic presidential party. “The secondary related issue is what division of the Democratic Party has the majority vote that will coalesce the party.”

While Warren has proven to be a strong fundraiser and places near or above former Vice President Joe Biden in most polls, it is unclear whether criticism from moderate candidates will affect her poll numbers — particularly because her ability to withstand attacks will be a test for a primary election against Trump, Henderson noted.

Endorsements are another metric important to displaying that a candidate can unite a party in an upcoming election, Henderson said. Warren is lagging in endorsements from party elites like current members of Congress, governors, and Democratic National Committee members compared to Biden, according to FiveThirtyEight’s endorsement tracker .

For some voters, considering electability is frustrating, but necessary.

“As much as I don't want to have to factor those things into my votes coming up, it's something that we do have to sit there and factor because, like, all these people that you would never suspect that would be so racist, homophobic, xenophobic, pick a phobia, have come out of the woodworks,” said Kate West, a 37-year-old pastor in Belle Plaine, Iowa.