Mark Jones is the chair of the Political Science department at Rice University. He says voters fall into the "undecided" category for two reasons. One, they're simply not paying attention.
"They don't really follow the news or politics very much, and they don't really have a really well-defined opinion regarding major political events."
The second reason for a voter to be undecided is more complicated. Jones says there are partisans who 'do' pay attention to what's going on, but are conflicted.
"They, at one level, want to support their party's candidate. But they 're dissatisfied with the person who actually is their party's candidate."
Jones says it's important not to confuse the "undecided" voter with the "independent" voter. He says the data from Gallup show the audience at tonight's town hall debate are mostly "not" independent.
"About 40 percent of them are Democrats. About 20 percent of them are Republicans. And about 40 percent of them have no political affiliation. So when we see these undecided citizens asking questions to candidates tonight, you shouldn't think these are independent people."
Most polls find roughly five percent of voters, nationally, are still undecided in the presidential race. Jones says that number has shrunk in recent election cycles, because of the increasingly polarized political environment. He says it's gotten to the point where it likely won't shrink much more.