It's a close race, but most polls say Elizabeth Warren is closer to winning it.
So, tonight, Scott Brown has to make a move up, while Elizabeth Warren has to hold him down.
Here are some do's and don'ts that could help them both get what they want from this debate.
Instead of just talking about the middle class being hammered, tonight Elizabeth Warren should be the hammer…and hammer home her message: Whom exactly does she see as middle class? Let's hear some specifics.
Another do: Ride the elephant. The elephant is the Republican Party, and Mitt Romney is the republican ringmaster. Warren should take every opportunity to link Scott Brown to the GOP.
A big do for Brown: Easier does it. Brown was wrapped pretty tight in the first debate, and he should loosen up for the second.
Another do: A barn coat with a brain. Brown's barn coat has taken him as far as it can. His challenge tonight is to show he's much more than a coat hanger in a pickup. In a word, Brown should be senatorial.
And now the don'ts.
For warren: Don't wilt. She doesn't always command the stage, so sometimes she appears weak. Tonight she should also be strong.
And don't repeat yourself. Warren does have a way of saying the same thing over again, and over again. The cure would be a new line of criticism, or more spontaneity. Both would make her better tonight.
For Scott Brown, don't be a bully. There's a very thin line between being aggressive and being a bully, and Brown has to be careful not to cross it.
Finally, don't underestimate Warren. She may be an inexperienced candidate, but she's also ambitious. Brown should expect a surprise attack at any time, so he won't look surprised when it comes. This is a debate where everything's possible.
Either candidate could win, or lose, and a lot of undecided voters are deciding.