It's Election Day in South Carolina!

(Bruce Smith/AP)

Republican voters in the Palmetto State's 1st district on Tuesday will choose a nominee in a runoff between former governor Mark Sanford and former Charleston county council member Curtis Bostic. We'll have results for you over on Post Politics tonight. Meantime, here are the five things worth watching as the returns come in. (Did we miss anything? The comments section awaits!)

1) Sanford's margin

Sanford enters the day as a heavy favorite to win. He's better-known, better-funded and has maintained a much more robust paid media presence. Sanford's margin of victory is really the only uncertainty at this point. He won 37 percent of the vote in the primary while Bostic carried just 13 percent. Sanford is expected to win the runoff by double-digits.

2) Bostic's reaction

If Sanford wins, it will be worth watching whether Bostic immediately endorses him. The race hasn't been that nasty, so there's no reason to think that Bostic would hold out, but politics never ceases to surprise us.

The underdog Bostic isn't a household name in the 1st district, but he bested 14 other candidates to advance to a runoff against a former governor with near universal name recognition. And he held his own in a televised debate last week, so he's cultivated some clout and his approval would mean something -- especially among his Christian conservative base. What's more, the last thing Sanford would need heading into the general election is discord within the GOP.

3) Turnout

There are a couple of competing lines of thinking here. On the one hand, if turnout is high it could be bad news for Sanford, operating under the belief that opinions about the former governor are so well-entrenched, he's not going to win many more than the roughly 20,000 votes he won last time. On the other, higher turnout generally benefits the candidate with the higher name ID, which in this case is Sanford. And if many new Republican voters turn out, Sanford's name recognition stands to be an asset.

For a reference point, turnout was about 53,000 two weeks ago.

4) Charleston, Dorchester and Berkeley Counties

Charleston County is where the most votes are up for grabs. Sanford's margin over Bostic there (41 percent to 13 percent) was pretty close to his overall margin over the former county council member (37 percent to 13 percent) two weeks ago. If Sanford is doing well here in round 2, it will be virtually impossible to stop him.

It's also worth keeping an eye on neighboring Berkeley and Dorchester Counties. Sanford easily won both in the primary, but Bostic did a little better there than he did in Charleston. If he's going to pull off a huge upset, he'll need to put up a big showing there on his second pass.

5) Elizabeth Colbert Busch (D) and the outside spending battle

Okay, this is really more a question for Wednesday morning, but it's worth tossing out there preliminarily. Polls have shown Colbert Busch running competitively against Sanford, even though the district leans heavily Republican. Would groups like the Democratic Congressional Campaign Committee and House Majority PAC join the fight on her behalf against Sanford? It's hard to see how they would not if the race starts as close as the numbers suggest. And in a close race, how heavily would Republican outside groups and House GOP leaders rally to Sanford's side, especially considering his 2009 fall from public grace? These are the questions we're going to begin asking if Sanford wins the runoff.