Much of the campaign work is taking place behind closed doors, with campaign teams battling tooth and nail to identify, register and ensure participation by potential supporters. Identifying potential voters is itself a moving target—but with mail-in voting about to begin, a first look reveals some interesting patterns.

As previously reported, the latest ABC News/Ipsos poll has Kamala Harris leading Donald Trump by nearly 4 percentage points (50-46%) among all adults and registered voters, and by 6 percentage points (52-46%) among likely voters. While these numbers are nearly identical, a closer look reveals a trend toward Harris in some groups when comparing all adults to likely voters—particularly among those under 40, particularly younger women and blacks.

That analysis, conducted by Langer Research Associates for ABC News, shows support for Harris rising from 54% of all adults under 40 to 64% of those identified as likely voters. Trump’s support, meanwhile, falls from 42% of adults in that age group to 33% of those likely to vote.

This change is being driven primarily by women: Harris’ support is rising from 60 percent of all women under 40 to 73 percent of those in that age group who are likely to vote. Trump’s support is falling accordingly, from 35 percent of all women under 40 to 24 percent of those who are likely to vote.

Harris has 82 percent support among all blacks, and that number rises to 93 percent among likely voters. Trump, on the other hand, rises from 15 percent support among all blacks to 7 percent among likely voters. Harris is also gaining support among liberals.

Countermovements in Trump’s direction do not reach a statistically significant level in any group. For example, he has 52 percent support among all 50- to 64-year-olds (his strongest age group) and 56 percent among likely voters in that group. His share rises to between 85 and 88 percent among conservatives and between 52 and 54 percent among people without a four-year college degree.

But it’s not just about who gets more support in the likely voting groups, but also the size of those groups. Harris, for example, is 23 percentage points ahead of Trump among likely voters with four-year college degrees, or 60 percent to 37 percent. She benefits from the fact that this group makes up 45 percent of likely voters, while it makes up only 35 percent of the population as a whole.

Trump can defend himself with his 10 percentage point lead among whites: they make up 61 percent of all adults, but 71 percent of likely voters. The smaller number of adults under 40 also works to his advantage: they make up 36 percent of all adults, compared to 25 percent of those considered likely voters.

All in all, it’s the combination of group size and the preferences of each group that keeps the race close. Who is likely to vote here is defined in part by people who say they’re certain to vote. Actually motivating them to do so is what both campaigns are about – starting now.

Editor’s note: This story has been updated to reflect that mail-in voting begins soon.