The Wisconsin Voter The Journal Sentinel's Craig Gilbert explores political trends in a purple state and beyond. SHARE Click to enlarge Election 2014

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Midterm elections are as much about which people vote as they are about how people vote.

And Wisconsin’s race for governor is a perfect case in point.

In recent weeks, Gov. Scott Walker has carved out a narrow lead over challenger Mary Burke among the most likely voters, even though the race hasn’t changed — and remains deadlocked — among registered voters overall.

When you pool together the two polls released by the Marquette University Law School in the past three weeks, Walker leads Burke by 5 points — 50% to 45% — among those who say they are “absolutely certain to vote.”

By contrast, Burke leads by 11 points — 47% to 36% — among those who say they’re less than certain to vote.

Walker’s job rating is positive — 52% approve, 46% disapprove — among the most likely voters. But it’s negative — 40% approve, 49% disapprove — among less likely voters.

This pattern is largely a byproduct of two factors, says Charles Franklin, the political scientist who conducts the Marquette polls and who aggregated his two most recent surveys for this analysis.

First, Republicans right now are slightly more enthusiastic than Democrats about voting and more likely to say they’re certain to vote.

Second, Republicans typically have a turnout advantage in midterm elections because of their demographic makeup, which is whiter, older, and wealthier — characteristics that are historically linked with higher rates of voting.

Walker’s lead among the most likely voters is, of course, much more significant than his deficit among less likely voters. But that deficit also gives his opponents an opening.

The central challenge for Democrats in this race has always been turning out as many of their “drop-off voters” as possible — those who are drawn to the polls in presidential years but frequently drop out in other elections. In the last presidential race in the state, almost 3.1 million people voted. In the last midterm, just under 2.2 million people voted.

Presidential-only voters are disproportionately nonwhite, lower-income and younger. When they do vote, they expand the electorate, and Democratic candidates tend to do better.

In Wisconsin, Democrats have won the past seven presidential elections voters and the past five U.S. Senate races that took place in presidential years. Republicans have won five of the past seven midterm elections for governor.

These turnout patterns are all the more important in a state as polarized as Wisconsin, where party loyalty is extremely high; very few voters are crossing over to vote for candidates on the other side; and there are few undecided voters to persuade.

The recent Marquette polls suggest just how much the size and makeup of the electorate will shape the outcome.

Walker leads by 5 points among likely voters, largely because likely voters as a group lean slightly Republican in these polls.

But among all registered voters, Walker and Burke are tied, as they have been for months.

Among the electorate as a whole, very little about this race has changed since last spring. Walker was at 46.1% in May among registered voters and was at 46.4% last week. Burke was at 45.7% in May among registered voters and was at 45.1% last week.

Walker’s job approval among registered voters hasn’t been below 47% or above 49% since March.

The gains Walker has made in recent weeks all came among those most likely to vote.

Among that group, he now has a net positive image (52% view him favorably, 46% unfavorably) and Burke has a slightly negative image (40% view her favorably and 42% view her unfavorably).

Walker’s narrow lead among the most likely voters is encouraging for Republicans and sobering for Democrats because it fits into a prevailing modern-day pattern: All things being equal, smaller, midterm electorates favor the GOP.

At the same time, all things are not always equal, and the GOP midterm advantage is not absolute. In the last midterm (2010), the electorate was disproportionately conservative and older and hugely favorable to Republicans. But in the midterm before that (2006), the electorate wasn’t quite as old, wasn’t nearly as conservative and leaned Democratic.

In Marquette’s polling, the race has bounced around more among likely voters this year than it has among registered voters, because of partisan shifts in enthusiasm and intention to vote. In Marquette’s July and August polls, likely voters actually leaned more Democratic than not-so-likely voters, and Democrats were slightly more enthusiastic about voting than Republicans.

Since then, Burke has had a rough spell on the campaign trail — caught up in a controversy over the authorship of her jobs plan — and that pattern has reversed itself.

The electorate this fall probably won’t be as Republican as the pool of likely voters in Marquette’s past two polls, since Republicans rarely outnumber Democrats in major Wisconsin elections.

But it will almost certainly be more Republican than the November 2012 electorate, which delivered single-digit victories for Democrats in presidential and U.S. Senate races.

The two most important questions may be these:

Will one side be more motivated than the other to vote? And how many of their “drop-off” voters can Democrats turn out for a non-presidential election?

In a race as close as this one, in a state as divided as this, it is easier for the campaigns to influence the outcome by turning out more of their own supporters than by changing people’s minds.

Follow Craig Gilbert on Twitter @WisVoter