Elizabeth Colbert Busch, the sister of comedian Stephen Colbert, has won the Democratic nomination for a South Carolina House seat by a landslide, while the former state governor Mark Sanford will progress to a Republican run-off.

Polling took place on Tuesday in a US congressional primary race that threw together a colourful cast of candidates including Colbert Busch, whose brother joined her on the campaign trail; Ted Turner's son; and a former governor of the state who once gave new meaning to "hiking the Appalachian trail".

The primary, to select nominees for South Carolina's first district congressional seat, drew a crowded field, with two contenders for the Democratic nomination and 16 for the Republican. Colbert Busch, a businesswoman who is on the staff of Clemson University, lived up to her status as favourite to secure the Democratic nomination and was boosted by her brother's campaigning. Comedy Central host Stephen Colbert, who rarely abandons his satirical conservative persona in public, told CNN's Jake Tapper on Monday: "I'm willing to, you know, break the jewel of my own creation to try to do something for her.

"I'm not worried about what it would do to me or my show to try to help her as myself – not as my character, to help her as myself. And you know, if people think that's not the right thing for me to do, I don't care. It's my sister, and I'm willing to help her."

In a segment on his show last week Colbert had joked: "As a broadcast journalist I am obligated to maintain objectivity. It doesn't matter that my sister is intelligent, hardworking, compassionate and dedicated to the people of South Carolina."

There were only two contenders for the Democratic nomination – the other was Ben Frasier – but 16 for the Republican one, reflecting the seat's status as a relatively safe Republican district. The special election is for a place in the House that was vacated last year when Governor Nikki Haley appointed Tim Scott to fill an empty Senate place.

The district includes parts of Charleston and the surrounding area, and is strongly Republican. In 2010 Scott was elected with 65% of the vote to 29% for his Democrat opponent; he was re-elected in 2012 by 62% to the Democrat's 36%.

Mark Sanford had been favoured by local media to top the Republican poll but was equally regarded as unlikely to secure the 50% needed to prevent a run-off on 2 April. Sanford is looking for a political comeback after his troubled time as South Carolina governor. In 2009 he disappeared for almost a week, telling his staff he was hiking the Appalachian trail. He later admitted that he had been in Argentina with his mistress.

He has since divorced but asked his former wife to run his campaign. She declined.

Sanford benefits in part from name-recognition, not just from his time as governor and the sex scandal but because he was congressman for the same district before he was elected governor. If he wins his old seat back it would be a remarkable rebound for a man who was derided in the media and by fellow politicians only four years ago.

Name recognition appears to be a hindrance for Ted Turner, son of the founder of CNN, Ted Turner. In interviews the younger Turner acknowledged that it was difficult to stand as a Republican given his famous father and the fact that Jane Fonda is his stepmother. Others in with a chance include Chip Limehouse, a state legislator. The final election will be held on 7 May.

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