TUNIS — Tunisia’s young democracy has endured many shocks since it emerged from the Arab Spring protests of 2011. But its presidential runoff this month may pose a new and embarrassing question: What happens if the winner is in jail?

In the first round of presidential voting last month, all the major-party candidates were knocked out, leaving two contenders: Kais Saied, a previously obscure law professor who claimed 18.4 percent of the vote as an anticorruption independent, and Nabil Karoui, a businessman and co-owner of a popular TV network, who won 15.6 percent.

Several polls before Sunday’s parliamentary elections showed Mr. Karoui’s new Qalb Tounes (Heart of Tunisia) party to be ahead. But with the presidential runoff a week away, Mr. Karoui is behind bars, awaiting trial on charges of tax evasion and money laundering.

“It is a unique situation in Tunisia,” Prime Minister Youssef Chahed said Friday in a television interview. As the presidential candidate of the Tahya Tounes party, he was eliminated in the first round.