Voters in the coastal 1st District hit the polls Tuesday to cast ballots in the Democratic and Republican special-election primaries.

Insiders expect more than 30,000 people to vote in the Republican primary; 16 candidates will be on the ballot.

The front-runner is former Gov. Mark Sanford. He is expected to be the top vote-getter but will likely fall well short of 50 percent. If that happens, he will land in an April 2 runoff with the Republican who receives the second largest share of votes.

Insiders believe that the most likely candidates to make the runoff with Sanford are attorney Curtis Bostic, state Sen. Larry Grooms, former state Sen. John Kuhn, state Rep. Chip Limehouse and economics teacher Teddy Turner, the son of media mogul Ted Turner.

But with so many candidates on the ballot and the margin for coming in second potentially so slim, it could be a late night.

On the Democratic side, Elizabeth Colbert Busch, the sister of comedian Stephen Colbert, is expected to win the primary outright. She faces frequent candidate Ben Frasier.

Polls are open from 7 a.m. to 7 p.m. Voters need to have a valid form of photo identification to cast a ballot.

The GOP-leaning seat became open when Rep. Tim Scott, a Republican, was appointed to the Senate to fill the vacancy caused by the resignation of then-Sen. Jim DeMint, also a Republican.

CQ Roll Call rates the 1st District race as Likely Republican. The special election will be held May 7.