Thursday's CNN/Univision Democratic Debate in Austin is arguably the most important of the primary season, certainly if you're a supporter of Hillary Clinton. The smart money says she needs to draw a very clear contrast from Barack Obama if she is to slow down his roll, but there's real danger for her, too.

We'll get an instant read on how Obama and Clinton do, thanks to a panel of undecided Democrats and Independents who have agreed to watch the debate with handheld dial testing units to register their likes and dislikes. They've been chosen to reflect the demographics of Texas Democrats.

We can't tell whether attacks hurt the intended target, since our testers can only react to the person doing the attacking. But we do know that the blowback against the attacker is often significant. Take a swipe at your opponent, and you can almost guarantee that the red line that charts how well you're doing will take a dive.

Because Senator Clinton might feel under pressure to be aggressive, but that would put her at risk of getting a negative response.

After the debate, AC360's Erica Hill will look at how our undecideds saw the debate, and whether Hillary Clinton, and Barack Obama, pulled it off.

-Steve Turnham, 360° Producer