New polls out of Iowa say Donald Trump and Hillary Clinton have solid leads, but both races may turn out to be closer than surveys suggest. Photograph by Joe Raedle / Getty

If the opinion polls are right, Donald Trump and Hillary Clinton will be celebrating victories in the Iowa caucuses, on Monday night. All of the voter surveys carried out in the past week have shown them in the lead. Of course, the polls could be wrong. Iowa is a notoriously tough place to poll, and much depends on how many new voters Trump and Bernie Sanders can get to the caucuses. But, the day before the vote, Trump and Clinton are the favorites to come out on top.

On Saturday evening, Des Moines Register/Bloomberg Politics released the final iteration, for this cycle, of the Iowa Poll, which is widely regarded as the best in the state. On the Republican side, it showed Trump leading Ted Cruz by five percentage points, twenty-eight per cent to twenty-three per cent, with Marco Rubio in third place, at fifteen per cent. On the Democratic side, the poll showed Clinton leading Sanders by three percentage points: forty-five per cent to forty-two per cent.

Even though the Iowa Poll has a good record, it is best not to place too much stress on any individual survey, especially when it is indicating, as this one does, that the races are tight. (The Iowa Poll has a standard error of plus or minus four percentage points.) But this latest survey is notable because its results are consistent with those generated by other recent polls, which have also shown Trump and Clinton in the lead.

In the past week, four pollsters have contacted Iowans—Des Moines Register/Bloomberg Politics, NBC News/Wall Street Journal/Marist, Public Policy Polling, and Gravis Marketing/One America News. On the Republican side, their surveys have shown Trump leading Cruz by five, seven, eight, and four points, respectively. On the Democratic side, they have shown Clinton leading Sanders by three, three, eight, and eleven points.

If you take simple arithmetic averages of these four polls, they show Trump getting 30.5 per cent of the Republican vote, Cruz getting 24.5 per cent, and Rubio getting fifteen per cent. On the Democratic side, the arithmetic shows Clinton at 48.5 per cent and Sanders at 42.3 per cent. So both Trump and Clinton are projected to win comfortably.

For a variety of reasons both races may well turn out to be closer than these numbers suggest. Indeed, it wouldn’t be a big surprise to see either Trump or Clinton lose. (The odds that they will both lose are a good deal longer.) As you’ve probably heard many times by now, it all comes down to turnout, which the pollsters can only make guesses about. If turnout is low, Cruz and Clinton are favored. If it’s high, Trump and Sanders are favored.

On the Republican side, Trump still seems to have the momentum, and Cruz’s support appears to be slipping a bit. You can see this by looking at the results from pollsters that carried out two surveys in January. The Iowa Poll showed Trump gaining six percentage points over the past three weeks and Cruz losing two points, while the latest NBC News/Wall Street Journal/Marist poll showed Trump gaining eight points and Cruz losing three points over a similar time period.

Cruz has to hope that some Trump supporters don’t make it to the caucuses, or that some non-Trump voters switch to him at the last minute in hopes of seeing the front-runner defeated. Indeed, many Iowa Republicans don’t like Trump. According to the Iowa Poll, his favorable rating among likely voters in the Republican caucus is just fifty per cent, and his unfavorable rating is forty-seven per cent. Cruz has a much higher favorable rating (sixty-five per cent) and a much lower unfavorable rating (twenty-eight per cent). “The drill-down shows, if anything, stronger alignment with Cruz than Trump, except for the horse race,” J. Ann Selzer, the pollster who carried out the survey, remarked.

It should also be noted that, in the past, the polls in Iowa have been most wrong when they have underestimated the influence of evangelical voters, who make up Cruz’s base. Conceivably, something like this could happen again. In the past couple of weeks, Cruz’s campaign has been hammering Trump on the airwaves for his past support of abortion rights. The Iowa Poll indicates that forty-seven per cent of the likely Republican electorate consists of self-identified evangelicals. In 2012, however, exit polls indicated that fifty-seven per cent of voters were evangelicals. Although Trump has recently made up some ground among evangelicals, they still, as a group, represent a threat to him.

The Democratic race appears to be a bit more straightforward, but it’s also tighter. Both Clinton and Sanders have extremely high approval ratings among likely Democratic voters, and both candidates’ supporters say that they are enthusiastic about their choices, with Clinton’s favorable rating at eighty-one per cent, according to the Iowa Poll, and Sanders’s favorable rating at eighty-two per cent. Early in January, Sanders appeared to be gaining ground on Clinton, and some polls showed him edging ahead. If the most recent polls are to believed, the Clinton campaign has succeeded in stopping Sanders’s momentum and preserving a narrow lead.

However, this basic picture disguises an epic battle to turn out the vote. As is now well known, Sanders’s electoral base is voters under the age of forty-five, who, historically, have tended to vote in lower numbers than older voters. Clinton is running much stronger with the latter demographic. For Sanders to win he has to get a lot of younger people who haven’t voted before to show up.

In many ways, Sanders is trying to repeat President Obama’s feat in Iowa in 2008, when first-time caucus-goers flocked to him. In 2004, about a hundred and twenty-five thousand people voted in the Democratic caucus; four years later, the figure rose to two hundred and twenty-seven thousand. Most analysts think that for Sanders to have a realistic chance of winning, the turnout this year will have to be close to two hundred thousand. And even then his precinct managers might have to persuade a majority of the supporters of Martin O’Malley, who is running at four per cent in the HuffPost Pollster average, to switch to him. (Under the arcane rules of the Democratic caucus, voters for candidates who don’t reach a certain numerical threshold can switch their allegiances after they have cast their first vote.)

Can Sanders do it, or will Clinton hold on? We will find out very soon.