As we enter the final week of the election, the Trump campaign is trailing in most polls and running out of time to make up ground in this election. After pulling advertising out of the upper Midwest in early October, the Trump campaign has abruptly changed course and is once again laser-focused on these same Midwestern states.

Throughout the year, the Elections Research Center at the University of Wisconsin-Madison has been surveying the same Midwestern battleground state voters. These surveys show President Trump was in trouble early. Potential swing voters strongly disapproved of Trump in February, even before the pandemic had spread widely in the United States. Joe Biden has been able to consolidate support from these voters, particularly during the past month. With Biden having a huge lead among those who have already voted, the question is whether Trump can muster enough support among Election Day voters to pull ahead of Biden.

How we did our survey

The Elections Research Center at the University of Wisconsin-Madison has been surveying the same Midwest battleground state voters throughout the year. We talked to these voters in February, August, September and October to see how their views have changed over the course of this race. Our latest poll was conducted online by the survey firm YouGov. It was fielded Oct. 13 to 21 with a sample of 800 voters each in Michigan, Pennsylvania and Wisconsin.

Biden is leading in states that were crucial to Trump’s 2016 victory

We find Biden for the first time taking sizable leads among likely voters in all three states. Biden leads Trump 52-42 in Michigan, 52-44 in Pennsylvania and 53-44 in Wisconsin. Biden’s lead stems from his ability to win over most undecided and potential third-party voters.

In our surveys, we identified three types of voters. One was Trump loyalists who never wavered in their support of the president and constituted 43 percent of the electorate. The second was Biden loyalists, who also made up 43 percent. The remaining 13 percent of voters were either uncertain whom they would vote for or shifted their view during the race. Biden is leading among this third group of “up-for-grabs” voters, 55 percent to 23 percent).

The seeds of Biden’s lead, and Trump’s troubles, were evident early on. Back in February, these up-for-grabs voters had already soured on Trump: giving him an abysmal -41 net approval score (the percent who approve minus the percent who disapprove). Only 19 percent of them thought the country was moving in the “right direction” even before the virus had spread widely in the United States. So those voters who were not ready to commit to Biden nevertheless had serious concerns about the president and about the direction of the country.

When did Biden consolidate his support?

There were two major inflection points when Biden appeared to consolidate support: after the Democratic primaries and during the crucial window between September and October. Between February and July, the number of undecided or third-party voters dropped and Democratic leaning voters shifted to Biden. Biden made further inroads between the September wave, which was fielded before the first debate and Trump’s covid-19 diagnosis, and this most recent survey.

What do we know about people who have and have not already voted?

Our survey shows large differences across the three states in the proportion of voters who report having already voted, although Biden leads among these voters in all three states.

In Michigan, 44 percent report having already voted, compared with 41 percent in Wisconsin but only 22 percent in Pennsylvania. In each state, the vast majority report voting by mail, with the rest voting early in person. Among early voters, Biden leads by a large margin: 75-23 in Michigan, 73-26 in Wisconsin, and 87-9 in Pennsylvania.

Among those who haven’t voted, Biden has a large lead among those who plan to vote by mail but a smaller lead among those who plan to vote early in person. For example, in Michigan, Biden leads 65-29 among the 13 percent who haven’t yet but still intend to vote by mail, and 58-42 among the 4 percent who intend to vote in person before Election Day.

But Biden trails badly among those who plan to vote in person on Election Day. In Michigan, 40 percent plan to vote in this way, and Biden trails 23-68. Biden faces similar numbers in Wisconsin. In Pennsylvania Biden is losing by a similar margin (31-67) but a much larger share of likely voters (57 percent) plan to vote in person on Election Day.

Thus, Trump’s best prospect among these three states appears to be Pennsylvania. It has the narrowest margin in our survey and polling averages, and only 22 percent of likely voters report having already voted.

What question marks remain?

Two issues make it hard to predict the ultimate outcome. First, there is added uncertainty about whether all mail ballots will be counted. On Monday evening, the Supreme Court’s conservative majority struck down a Wisconsin measure that would extend the period for counting ballots that were postmarked by Election Day but arrived later. This could affect the six percent of Wisconsin likely voters who plan to vote by mail but haven’t yet.

A similar issue may arise in Pennsylvania. Right now, ballots received within three days of Election Day will be counted. But Republicans are pushing the Supreme Court to revisit an earlier case in which the court deadlocked 4-4, allowing the three-day grace period to stand. Biden leads 70-26 among the 13 percent of Pennsylvanians who still plan intend to vote by mail.

Finally, many states, and especially Wisconsin, are facing a severe covid-19 outbreak. This could make Biden’s lead in early voting especially advantageous if the outbreak dissuades some voters from showing up to vote in person, or drives some voters away from Trump. But ultimately, it is hard to know. Trump’s 2016 victory was hardly expected and certainly this year’s polling, including ours, cannot rule out another surprise, however unlikely it may be.

Eleanor Neff Powell is Booth Fowler Associate Professor of Political Science at the University of Wisconsin-Madison.