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The persuadable voters: Who's up for grabs tonight?

Clinton speech preview

PHILADELPHIA -- It's a small number: fewer than one in ten voters in the battleground states say they're truly undecided, in that they won't even choose a candidate in a poll. And they're also less likely to vote in the first place. It's only another 15 percent of swing-state voters not currently with Clinton who'd still consider voting for her right now - and most of them say "maybe," at best, they'd consider her.

How Hillary Clinton is appealing to women voters and the middle class

So it's that consideration number she would need to change first; to persuade people to take another look at her, and only then might she later persuade them vote for her. (This construct is similar on the other side for Trump, too.)

A quick look at voters like this whom we've surveyed recently in our Battleground Tracker shows they tend to be younger, under 45 - so many of them won't have much political memory of the 1990's and, hence, it is no accident the Democrats have spent a lot of time trying to reintroduce Clinton here. They say they don't pay as much attention to politics, which is a profile typical of uncertain voters, more generally - and it leaves open the question of whether they'll even be watching tonight.

They're mixed on whether they agree with Donald Trump on key policies like immigration, which may suggests this might come down more to a personal, rather than a policy decision, and this would also be fairly typical for undecided voters. People who hold hard policy preferences tend not to waiver as much on their candidate choices. But they do agree with Trump on the need to change Washington, which suggests Clinton has to convince these voters, at minimum, that she could also bring change. And they're uneasy about the direction of the country, so Clinton needs to address that anxiety as well.

It's also no accident the Democrats haven't railed as much against the Republican party as much as they've gone after Trump specifically: they'll hope there's a door open for a few Republicans disappointed with Trump. But while such disappointed Republicans do exist -for instance, Trump has had the hardest time convincing those John Kasich primary voters to back him now - and that's one reason he trails in places like Ohio. So far, the polling doesn't show these disappointed Republicans willing to vote for Clinton en masse. Thus far, they've indicated they'll vote third party, or not at all.

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