Former governor Mark Sanford took a step closer to returning to elected office Tuesday, advancing to a Republican runoff in South Carolina's 1st District special election.

With all votes tallied, Sanford led the field of 16 with 37 percent of the vote, a strong showing considering the crowded contest. Former Charleston county councilor Curtis Bostic narrowly won the second spot in the runoff, edging out state Sen. Larry Grooms by fewer than 500 votes, a margin slim enough to trigger a mandatory recount. Grooms strategist Hogan Gidley said late Tuesday the state senator was not planning to concede to Bostic.

Sanford will face the second place finisher in an April 2 runoff election, since no candidate received at least 50 percent of the vote Tuesday.

In the Democratic race, Elizabeth Colbert Busch, the sister of Comedy Central host Stephen Colbert, easily captured the nomination against perennial candidate Ben Frasier.

The vacancy in the heavily Republican district was triggered by GOP former congressman Tim Scott's departure for the Senate earlier this year. After Sen. Jim DeMint (R) abruptly announced his resignation from the Senate, Gov. Nikki Haley (R) appointed Scott as his replacement.

Sanford, a onetime rising national Republican star, who some even viewed as a budding presidential candidate, fell from grace in 2009 after disappearing for nearly a week and admitting to an extramarital affair. He is now engaged to the woman.

He's back two years later, trying to reclaim a seat he once held. Sanford represented the 1st district for six years before becoming governor. His near universal name recognition and reputation as a fiscal conservative boosted him ahead of a crowded field of GOP contenders.

Bostic, who headed into Tuesday's race as an underdog, is a former city councilor who lives outside the boundaries of the 1st district. Grooms is a conservative sate senator who was endorsed by Republican Reps. Mick Mulvaney and Jeff Duncan.

Whomever emerges as the Republican nominee will be a heavy favorite in the May 7 election. Nearly six in 10 voters there went with Mitt Romney in the 2012 presidential election.

In the Democratic race, perennial candidate Frasier was no match for the well-funded Colbert Busch, who dominated, with 96 percent of the vote. Addressing supporters, Colbert Busch made a broad appeal for voters of all parties.

“I want to say to the voters of the 1st Congressional District of South Carolina, Democrats, Independents and Republicans, I have been listening and I hear you," she said.

Colbert Busch has been a productive fundraiser, thanks to some help from her brother, who portrays a faux-conservative commentator on Comedy Central. Colbert has appeared at fundraisers for his sister, and mentioned her campaign both in character on his show, and out of character in interviews.