Updated at 9:48 p.m.

Former governor Mark Sanford won a Republican runoff election in South Carolina’s 1st congressional district on Tuesday, advancing to a head-to-head race against Democratic nominee Elizabeth Colbert Busch that early indicators suggest will be competitive.

The ex-governor defeated Curtis Bostic, a former Charleston County Council member, 57 percent to 43 percent.

Sanford was widely expected to defeat Bostic, whose campaign was dwarfed by the better-known and better-funded former governor. “It certainly has been a David and Goliath race,” Bostic said in a Monday interview.

Since no candidate received more than 50 percent of the vote in the March 19 GOP primary election, Sanford and Bostic advanced to a two-week runoff campaign. Sanford took 37 percent of the vote two weeks ago, while Bostic edged into the runoff with 13 percent. Fourteen other candidates were on the ballot.

The ensuing runoff campaign was pretty quiet. Bostic sought to energize his Christian conservative base and both he and Sanford kept up fiscally conservative messages. The two candidates engaged in a debate for the first time last Thursday, an hour-long event notable for Sanford addressing his dramatic 2009 fall from grace.

“I failed very publicly,” declared the former governor, who disappeared for nearly a week as governor before admitting to an extramarital affair.

Sanford now faces his toughest challenge yet in his bid for a return to elected office. Colbert Busch, the sister of faux conservative commentator Stephen Colbert, has been steadily building name recognition and a campaign war chest. Slowly, she's been winning the confidence of Democratic donors and national strategists.

“I look forward to a vigorous campaign that focuses on creating jobs, balancing our country’s budget and choosing an independent-minded leader who shares the values of the great people of South Carolina,” Colbert Busch said in a statement her campaign released shortly after Sanford won the GOP nomination.

The 1st district, which hugs the Atlantic Coast from Hilton Head Island north through Charleston, is a very conservative area; Mitt Romney won nearly six in ten votes there in 2012. But polls show Colbert Busch, who’s spent part of her career in the shipping industry, begins the race with a real chance of pulling an upset. What's more, Sanford’s baggage makes him a ripe target for attack ads ahead of the May 7 election.

"This is the same Mark Sanford who, as governor, disappeared from office and used taxpayer money to visit his mistress," wrote Sen. Kirsten Gillibrand (D-N.Y.) in a fundraising e-mail to supporters, encouraging donations to Colbert Busch.

Republicans, meanwhile, will likely try to cast Colbert Busch as a liberal, and tie her to national Democratic interests. So far, she hasn't faced any real attacks. Her primary was not competitive and the extended GOP nominating process occupied the lion's share of Republican attention.

The special election was triggered earlier this year when then-Rep. Tim Scott (R) was appointed to replace Republican Jim DeMint in the Senate. DeMint resigned his seat to head a conservative think tank.

“South Carolina families clearly deserve a continuation of the strong fiscal discipline displayed by now-Senator Tim Scott and will greatly benefit from Sanford’s fiscal hawk approach," said National Republican Congressional Committee Chairman Greg Walden of Oregon, in a statement.

Sanford represented the 1st district from 1995-2001, when the district lines were a bit different. He served two terms as governor through 2011.