DES MOINES, Iowa -- Sheila Campbell was not a natural door-to-door campaigner. The Democrat from Urbandale, Iowa, canvassed for one day in 2008 for Barack Obama and hated it so much that she didn't do it again.
Until this year.
Since early April, Campbell, an office manager for a neuropsychology clinic, has been knocking on doors and talking up Sen. Bernie Sanders to Democrats, Republicans, independents -- to anybody who would listen. The day the Vermont senator had a heart attack in October, she went to a campaign office to volunteer more hours.
"This man is putting everything on the line for us," Campbell said of Sanders. "Whatever I can do, if they call, if they ask, I say 'yes.'"
With less than three months until the Democratic presidential caucuses in Iowa, the Sanders election machine is powered by the renewed zeal of the senator's followers in the aftermath of his heart attack and by the endorsement of star freshman Rep. Alexandria Ocasio-Cortez of New York.
Some recent Iowa polls have shown Sanders trailing the front-runner. His campaign is hoping to gain ground by relying on the strong commitment of the senator's base to draw in first-time caucusgoers who are primarily younger or working class.
"I think people would make a mistake to underestimate the enthusiasm that is still out there for Bernie Sanders," said Democratic state Sen. Rob Hogg of Cedar Rapids, who is undecided in the race. "I think he's turned a little bit of a corner, and just as I watch, I think he's got some momentum now that he didn't have three months ago."
A CNN/Des Moines Register/Mediacom poll released Saturday shows Mayor Pete Buttigieg of South Bend, Ind., as the latest candidate to take the lead in Iowa, pushing past Sen. Elizabeth Warren of Massachusetts, former Vice President Joe Biden and Sanders. All four are poised to meet the 15% threshold of support required to win a share of Iowa's delegates in the Feb. 3 caucus that starts the presidential nominating process.
The crowded field has diluted the following Sanders built in 2016, when he shocked the political world by fighting the more moderate Hillary Clinton to a draw in Iowa. Many former supporters also have raised concerns about the age of the 78-year-old senator.
That, along with the fact that Sanders turns off more centrist voters for being too left-wing, has led some experts to discount Sanders' prospects.