"No free airtime, Lulu," Colbert said, referring to his sister by her childhood nickname. "As a broadcast journalist, I am obligated to maintain pure objectivity. It doesn't matter that my sister is intelligent, hardworking, compassionate and dedicated to the people of South Carolina."
Colbert-Busch, who is running as a Democrat, faces an uphill climb in the GOP-leaning district. President Obama won just 40 percent of the vote there last year.
Colbert offered some advice to Republicans who might end up in a debate with his sister. "This is how I used to win a lot of arguments with her when I was a kid," he said. "Right after she makes a good point, repeat it back to her in a dumb voice."