And then there were three

Mark Sanford, flanked by sons Marshall (from left), Blake and Bolton, celebrates a first place finish in a field of 16 candidates in the Republican primary. Though not an outright victory, it ensures him a spot in the party's runoff to face Elizabeth Colbert Busch in the 1st District congressional race to fill Tim Scott's seat.

A 24-point election margin sounds like a landslide. But in a primary that requires a majority winner, a candidate who finishes first with 50 percent of the vote or less merely advances — along with the runner-up — to a runoff.

So supporters of both the top two finishers in Tuesday’s 1st Congressional District primary shouldn’t assume that Mark Sanford’s 37-13 percent “victory” over Curtis Bostic makes this “Comeback Kid” a shoo-in for the GOP nomination when voters return to the polls on April 2. (State Sen. Larry Grooms finished a close third.)

Nor should Republicans assume that their party will automatically extend its 16-term hold on this seat.

After all, the Democrats also held a 1st District primary on Tuesday, with Elizabeth Colbert Busch rolling past Ben Frasier with more than 95 percent of the vote. She will oppose the GOP nominee in the May 7 general election to determine who replaces Tim Scott, promoted to the U.S. Senate early this year after Jim DeMint resigned with four years left in his second term.

Mrs. Colbert Busch, director of business development for the Clemson Restoration Institute on the former Charleston Naval Base, can find hope in this relatively recent 1st District outcome: In 2008, Republican incumbent Henry Brown defeated Democratic challenger Linda Ketner by only four percentage points.

As for the looming GOP runoff, Mr. Sanford’s own electoral record proves that finishing second in the first round doesn’t preclude winning the second round:

In the initial 1994 1st District GOP primary, Mr. Sanford got only 19 percent of the vote. But in a seven-person race, that was enough for second place behind Van Hipp Jr.’s 30.8 percent.

Then Mr. Sanford won the runoff by 52-48 percent.

In the initial 2002 Republican gubernatorial primary, Mr. Sanford edged Bob Peeler by 38.6-37.6 percent in another seven-candidate field. Then Mr. Sanford thumped Mr. Peeler, 60.1-39.9 percent, in the runoff.

Still, that doesn’t guarantee another big runoff gain for Mr. Sanford in 2013. As a three-term congressman and two-term governor, he had a considerable name-recognition edge over his 15 GOP opponents Tuesday.

Yet while 37 percent of the voters in the Republican primary chose Mr. Sanford, 63 percent did not.

And few South Carolinians have forgotten the notorious 2009 “Appalachian Trail” scandal, which made him a national laughingstock.

That memory should give Mr. Bostic, a former Charleston County Councilman, a good chance at gaining many votes from those who cast ballots for other GOP candidates on Tuesday.

Such political speculation, though, doesn’t decide elections.

Voters do.

Tuesday’s primaries thinned the 1st District field to three.

So if you voted Tuesday, your self-governing task in deciding who represents you in the U.S. House isn’t done.

And if you didn’t vote Tuesday, you can still have a say in this crucial choice by casting ballots in the runoff and general election.