Voters head to the polls in 13 days to choose between Mark Sanford and Elizabeth Colbert Busch in South Carolina's 1st district. The wildly unpredictable campaign has led to a scenario in which Colbert Busch has put herself in a good position to upset Sanford.

But Sanford is not finished just yet, given the conservative tilt of the district (Mitt Romney won it by 18 points in 2012.) Here are the four biggest things to watch during the stretch run for clues about who will have the upper hand on May 7:

(Mic Smith/AP)

1. The April 29 debate: This is likely the one and only chance voters will get to compare Colbert Busch and Sanford side by side before Election Day. The Sanford campaign has been complaining that Colbert Busch has been flying under the radar and running a "stealth campaign." This will be the former governor's clearest opportunity yet to engage his opponent. And for Colbert Busch, it's an opportunity to introduce herself to the voters who are considering supporting her but don't know much about her. And, of course, there is the elephant in the room: Sanford's 2009 fall from grace and public split with his wife, in addition to her recent allegations that he trespassed on her property. All of which is to say that this is debate is must-see TV for political junkies.

2. Any more surprises? The recent revelation that Sanford's ex-wife accused him of trespassing on her property surprised many people who thought all of his dirty laundry had been aired already. And Sanford's bungled response to the whole thing only made matters worse. Colbert Busch has since built a nine-point lead over Sanford, according to a recent poll from automated Democratic pollster Public Policy Polling. Any more last minute surprise revelations regarding Sanford would effectively end the race.

3. Vice President Biden: Just days before the special election, Biden will be in South Carolina to speak at the state Democratic Party's Jefferson-Jackson Dinner on May 3. Colbert-Busch's campaign called the timing of the vice president's trip "coincidental," adding that he won't be coming into the 1st district. Of course, that doesn't mean Biden can't decide to make a last-minute campaign stop or two on Colbert Busch's behalf. If he does, it will suggest Colbert Busch has a real chance of victory. If he doesn't, it probably means her odds are not that great.

4. National or local? The Democratic attack ads against Sanford have centered on his record in South Carolina -- the ethics fine he paid, and the first-class and business-class trips he took on the state's dime. The Republican strategy against Colbert Busch has been to tie her to national Democrats; the Sanford campaign calls House Minority leader Nancy Pelosi (D-Calif) Colbert Busch's "biggest benefactor" and slam her for taking union money. Will voters in the very fiscally conservative district be more bothered during the final 13 days by the prospect of electing someone who will join the Democratic caucus or Sanford's personal troubles?