If Mark Sanford is going to pull off a political comeback culminating in a return to elected office, he’ll need the next seven days to go smoothly.

Make no mistake, Sanford is the frontrunner for the GOP nomination in South Carolina’s 1st district special election. But the unpredictability of a special election runoff means he’s not a lock.

A week from Tuesday, Republican voters will be presented with two choices, as different as can be. In one corner is Sanford, a very well-known former governor who cultivated a popular, fiscally conservative reputation before falling from public grace in 2009. In the other stands Curtis Bostic, a largely unknown attorney and former county legislator with a small but loyal conservative following.

The differences don’t end there. Sanford won 37 percent of the vote in last week’s primary; Bostic barely edged into the runoff with 13 percent support. Sanford had more than $270,000 to spend as of the mid-March; Bostic had only about $57,000 in his campaign account. Bostic, notably lives just outside the 1st district boundary.

Neither candidate has gone negative against the other over the airwaves. Sanford’s portrayed himself as a strong fiscal conservative and implicitly asked voters for forgiveness in his TV ads. While he doesn’t explicitly mention it, Sanford's disappearance from the state in 2009 and admission to an extramarital affair is what he’s implicitly asking voters to forgive.

For all their differences, Bostic and Sanford are both taking a hard line when it comes to reining in federal spending. In an introductory campaign video, Bostic reaffirms his commitment to reducing spending and brandishes the political outsider card. “I’m no career politician,” he says in the video.

Bostic’s also built a following among Christian conservatives. He and his wife built a retreat area that churches and community groups regularly use, the Charleston Post and Courier reported. Bostic, a personal injury attorney, also founded a non-profit that directs aid to struggling countries.

Rep. Mick Mulvaney (R), who backed state Sen. Larry Grooms (R), the third-place finisher in the primary, said that he has no relationship with Bostic, but noted his enthusiastic conservative political base.

“I don’t know the other guy at all,” said Mulvaney, of Bostic. “Here’s what I do know about it. His support base is with the homeschoolers and the Christian right. And those folks are extraordinarily dedicated, and they will show up on April 2.”

Mulvaney said he intends to stay neutral in the runoff.

Turnout was higher than expected in last week’s primary, but compared to a regular election, it was pretty low. Next week’s runoff is expected to attract even fewer voters. For Sanford, the challenge is to rally his strongest supporters and compete for the portion of the electorate that didn’t vote for him or Bostic.

“Given the results of the primary, we feel like we are in good position to win the runoff,” said Sanford spokesman Joel Sawyer. “That being said, you can’t take anything for granted, particularly in a runoff scenario.”

Several of the 16 candidates who finished with minimal support in last week’s primary have chosen sides. Shawn Pinkston, Elizabeth Moffly, Keith Blandford and Jeff King lined up behind Sanford last week. Ray Nash announced his support on Monday. Together, they accounted for only about seven percent of the vote last week. John Kuhn, who won about six percent of the vote, is backing Bostic.

Given Sanford’s name-recognition, and his financial and organizational advantages, the path to victory for Bostic is narrow.

"I have a very short window, and only so much opportunity to introduce myself to voters, and I think what is most important is that they're aware of my strong history of being a fiscal conservative," Bostic told The Fix.

When asked if he would be pointing to Sanford's past, Bostic underscored that his focus would be on his own message of fiscal conservatism.

One GOP strategist argued that there is little that Sanford can do to move the needle this week with undecided voters.

“There’s just not a whole lot new that Sanford can do or say to convince undecided or unconvinced voters to support him,” said South Carolina Republican strategist Walter Whetsell, who regards Sanford as the frontrunner  Whetsell’s firm worked for Kuhn in the primary.

With a longer runoff campaign, things might be looking a little rosier for Bostic. But a week is not a lot of time to rally enough support to take down a well-known former governor in a district he once represented.

There is one other factor to keep an eye on as the runoff kicks into full gear this week: electability. Mulvaney said he’s convinced that Democrats want Sanford to be the GOP nominee against Elizabeth Colbert Busch, the sister of Comedy Central host Stephen Colbert. It's worth watching whether either side brings up the topic of who is more electable.

Indeed, Sanford’s past raises questions about his viability in a general election, particularly among women. The 1st district leans heavily Republican (Mitt Romney won nearly six in ten voters there last November), but Colbert Busch will have the money and name recognition to compete more heavily than most Democrats.

The GOP primary has long been Sanford’s race to lose. It still is. But special elections -- particularly runoffs -- can be unpredictable. That means the next week is a crucial one for Sanford.