NEWTON, Iowa—These are not easy times for a Democratic canvasser, and Cindy Pollard knows it. For months, she has been working the neighborhoods of this hollowed-out former manufacturing town east of Des Moines, driving up and down its streets in her black Toyota 4Runner. The vanity plate says "CINDY." One sticker says, "I'll believe corporations are people when Texas executes one." Another says, "I (Heart) My Wife."

At one house Pollard visits on an evening last week, a man shakes his head and says, "Just go along." At another, a woman says, "Technically, yes, I'm a Democrat, but I don't feel like we've got a good selection this year." In a sketchy apartment complex after dark, the man who answers the door yells inside, "Momma, some people here want to talk to you about voting," only to turn back to us and shake his head: "Naw, she don't want to talk to you." These are the people the Democrats have identified as the midterm voters they need.

Pollard, a stocky 57-year-old with gel-spiked blond hair, wears jeans and a purple T-shirt for Senate candidate Bruce Braley. She doesn't get discouraged, but she can't help but notice. "It does seem like we're getting less than at the beginning"—less people willing to hear her pitch for Democratic candidates, less people agreeing to send in their mail ballots, she says. "We're really picking it clean."

Democrats have long contended that volunteers like Pollard, massed into a fiercely disciplined army and deployed in unprecedented numbers across states like this, will help them beat the odds in this difficult election year. But so far, the results look discouraging. Iowans have been voting for more than a month already—they can vote early in person at polling places or by requesting and returning a mail-in ballot—and it is Republicans who are encouraged by the numbers.

Early voting, which may account for as much as half the total Iowa vote, has traditionally been a Democratic strength. This year, for the first time in Iowa history, that changed. As of last Wednesday, registered Republicans accounted for more early voters than registered Democrats, by a margin of 305 ballots—a tiny edge, but the first time the GOP had ever led in early votes. The lead has since been reversed, and as of Sunday, 124,000 registered Democrats and 122,000 registered Republicans had voted.

But that Democratic edge pales in comparison to past elections. In 2012, when President Obama won the state, about 67,000 more registered Democrats than Republicans voted early. Even in 2010, when the state's Democratic governor was getting creamed by his Republican challenger, about 20,000 more Democrats than Republicans voted early in Iowa—but more Republicans voted on Election Day, erasing Democrats' early advantage. (These statistics don't take into account independent voters, and they account only for which party voters register with, not which they actually vote for. But both parties watch them closely as an indicator of their mobilization efforts.)

Iowa Republicans worked for this result—the state party has poured $1 million into early-vote activation—but they are actually a little shocked to see this much success. "I wish I could tell you this was the master plan, to actually pull ahead in the early vote," Jeff Kaufmann, the mustached community-college professor who chairs the Iowa GOP, tells me. "I would have been happy if we were only behind by 20,000." If Democrats fail to produce either a massive late surge of early voters or an unprecedented Election Day turnout, it is rapidly becoming mathematically impossible for them to win the state, Republican operatives say. In 2010, the GOP won Election Day by 9 percentage points. "The only way Joni loses is if the Democrats are doing something we can't see," David Kochel, a strategist for Braley's opponent, Joni Ernst, tells me.

Officially, Democrats say they are not concerned. "We feel we have a significant advantage on the ground and we're winning early voting," says Christina Freundlich, spokeswoman for the Iowa Democrats. According to the party's data, she says, Republicans are merely convincing people to vote early who would have otherwise voted on Election Day, while Democrats are getting people to vote who otherwise might not have voted at all—people who don't have a track record of voting in midterm elections.

If this is true, by the time Election Day rolls around, Republicans will be at a disadvantage, because their people will have already voted. But the numbers Freundlich cited for this contention didn't show an enormous gap in Democrats' favor. Just 14 percent of Republican early voters didn't vote in 2010, she said, while 20 percent of Democratic early voters didn't vote in the last midterm. That's not a huge difference. Similarly, Democrats claim that, based on their targeting, the unaffiliated voters who've cast ballots early are overwhelmingly Braley voters, but the relatively small proportion of independents who vote early—about 55,000 so far—makes this a limited advantage as well. Overall, polls show neither candidate with a clear lead among independent voters.

Democrats have invested hugely in their ground game this year, hoping to replicate the vaunted Obama organization on a state-by-state scale. Nationally, the party is spending $60 million to hire 4,000 staffers across 10 states. In Iowa, Democrats have 35 field offices to Republicans' 13. "Winners Walk! Losers Talk!" says a poster on the wall of the one in Newton, which smells strongly of the Panda Garden Chinese restaurant next door.

But even at its best, experts say, the ground game only moves the vote a couple of percentage points. And when everything else is against you—an unpopular president, a sour national mood, a difficult midterm landscape, underperforming candidates—an army of phone-callers and door-knockers, backed by state-of-the-art targeting and modeling, still may not be enough to overcome the obstacles. You can't get people out to vote who don't want to vote.

Cindy Pollard knows her territory so well she sometimes knows voters' names before she looks at her clipboard, with its printed, street-by-street list of addresses. The first thing she does when she gets the list is throw away the map that comes with it. She's been told that, with more than 700 ballot requests to her name, she's the party's top Iowa volunteer. She knows which houses you have to approach by knocking on the garage instead of the door, because the men hang out there drinking beer. She's been yelled at, attacked by dogs, and asked how much it costs to vote.

A retired nurse and Iowa native, Pollard moved here from Des Moines 15 years ago to be with her longtime partner—now her wife. She spent years canvassing for anti-gay-marriage Democrats, believing they would one day come around, and it finally happened. She worked for Hillary Clinton in 2008 and can't wait to do it again in 2016.

And Pollard is sure she is making a difference. In the dark, sketchy apartment complex, where the stairwells' dirty walls radiate old cigarette smoke, she knocks on an upstairs door and it opens. Samantha Montgomery, a 25-year-old in sweat shorts and bare feet, seems ambivalent at first. She says she's an independent. She's a single mom, on her own, just got a job at a printing shop after a long time looking. High School Musical 2 is paused on the TV in the small living room behind her. She says her income is "below poverty" and she still owes $26,000 on the two-year degree she didn't finish. The election is on her birthday.

"Democrats are the way to go!" Pollard says. She talks about how easy it is to get a ballot in the mail.

"I do think I'll probably vote for Braley," Montgomery finally says. "Not saying anything bad about Joni Ernst, I think he's got a better idea what we should do here in Iowa." She begins filling out a ballot-request form.

I ask Montgomery if she would have voted if Pollard hadn't come here. "I'll be honest, I probably would have forgotten," she says. But now she is determined. "I think this really matters," she says.