Newsletter: Essential Politics: How to make sense of those conflicting Iowa polls

Sanders winning? Clinton ahead? Trump gaining? Cruz takes the lead?

We’re in the “pick your poll” phase of the presidential campaign, when surveys stream in like snow in a northeast blizzard. But it’s possible to make sense of the conflicting numbers.

Hello and good afternoon, I’m David Lauter, Washington bureau chief. Welcome to the Friday edition of Essential Politics, where we examine the events of the last week on the presidential campaign trail and look at some stories that provided insight beyond the daily headlines.

So, how to read those polls: A key difference among them is who they define as a likely voter. The number of people who actually show up to vote in a primary or caucus is a small percentage of the electorate. In Iowa, for example, turnout for the state’s caucuses seldom goes above about 20% of the number of registered Democrats or Republicans, with the Obama surge of 2008 a notable exception.

That creates a tough job for pollsters trying to figure out which voters to count. Pick only those who have shown up in the past, and you’ll clearly miss some newcomers. Take all those who claim they plan to vote, and you’ll surely count too many.

How tight a screen one uses matters greatly for the results. Hillary Clinton and Ted Cruz lead in Iowa among people who have voted in the past. Sen. Bernie Sanders and Donald Trump lead among those who say they plan to vote for the first time, including many young people in Sanders’ case and disaffected voters in Trump’s.

Not surprisingly, the CNN poll that came out Thursday showing Sanders and Trump way ahead in Iowa used a very loose voter screen. Polls that have shown Clinton and Cruz with strong leads used much tighter ones.

What’s that tell us? Sanders and Trump can win Iowa, but to do so, they’ll have to turn out large numbers of first-time voters. Whether they can do that remains among the biggest unanswered questions of the campaign -- one which we’ll have an answer to in just over a week.

We have a team of reporters in Iowa to keep you up to date. Follow them on our Trail Guide blog for the latest developments and on our Politics page for more in-depth reports.

And here are some stories about the campaign that are well worth your time over the weekend if you haven’t already read them:

Mark Z. Barabak traveled to Marshalltown, a small town in central Iowa, and talked to people there about the roiling campaign debate over immigration. What they said provides an important reminder that voters’ views are usually much more nuanced than the loud proclamations of candidates on the stump.

New Hampshire, which holds its first-in-the-nation primary eight days after Iowa’s caucus, prides itself on its easy access to the ballot. This year, as in past elections, scores of unknowns will share the primary ballot with better-known names like Clinton, Trump and Jeb Bush. Michael A. Memoli moderated a debate this week with 23 of the lesser knowns. His account of the experience is a tale of grass-roots democracy persisting in the media age.

Latinos make up one of the fastest-growing groups in the American electorate and one that could be pivotal in this election. Kate Linthicum had two stories this week about the country’s Latino population. One looked at the rapid growth of the potential Latino electorate and why actual turnout likely will continue to lag behind that potential. The other presented the latest evidence on a fact often ignored on the campaign trail: the number of immigrants in the country illegally has been declining rapidly over the last decade.

Then there are the candidates. Seema Mehta examined Sanders’ surge. Chris Megerian took a good look at New Jersey Gov. Chris Christie, whose campaign revolves around his personality.

“I’m like a fungus,” he says. “I grow on people.”

The two of them teamed up for a look at Sen. Marco Rubio of Florida, whose goal in Iowa is coming in third.

Cathy Decker, Evan Halper and Mehta anchored our coverage of Sunday’s Democratic candidate debate. And Kurtis Lee looked at how Sanders and Clinton are aiming appeals at black voters.

Finally, amid the welter of campaign promises, do you wonder what the public sees as the top priorities for the government? The nonpartisan Pew Research Center surveyed Americans, and here’s our account of what they found out.

What we’re reading

Can Rubio swing the nomination even if he loses the first few contests? What percentage of the delegates would Trump have to amass to become unstoppable? Where does Cruz need to prevail? David Wasserman of the Cook Political Report teamed up with the data experts at FiveThirtyEight.com to answer those questions. The result is an invaluable map to the contests ahead. On the Democratic side, Wasserman also makes a telling point: Iowa and New Hampshire are must-wins for Sanders; Clinton, by contrast, could definitely win the nomination without them.

That wraps up this week. My colleague, Christina Bellantoni, will be back Monday with the weekday edition of Essential Politics. Until then, keep track of all the developments in the 2016 campaign with our Trail Guide, at our politics page and on Twitter at @latimespolitics.

Send your comments, suggestions and news tips to politics@latimes.com.