SA Best leader Nick Xenophon has challenged Steven Marshall to resign as Liberal leader if his party fails to win enough seats to form majority government at the March 17 election.

Mr Marshall and Mr Xenophon questioned each other in the ABC Radio Adelaide studio this morning during the first organised debate of the as yet unofficial campaign.

Premier Jay Weatherill also joined the radio debate from Port Pirie, where he is campaigning as part of a week-long regional tour.

Mr Marshall has publicly ruled out negotiating with Mr Xenophon and SA Best to form government should the Liberals fall short of the 24 seats required to govern in their own right.

"Given the very remote possibility that you will hold 24 seats, will you now announce that you will resign as Liberal leader if, on the 18th of March, you're nowhere near 24 seats?" Mr Xenophon asked the Liberal leader.

"Or will you sit down and negotiate with us as we want to with both sides after the election?"

Mr Marshall declined to answer directly about whether he'd resign.

"You've made it very clear you're on the Labor Party's side," he responded.

Mr Marshall asked Mr Xenophon to rule out forming government with Labor.

The SA Best leader said that while he agreed that Labor had done a number of bad things for the state, it was too early to say who SA Best would offer support to.

"It depends on the will of the electorate. It depends on how many seats SA Best wins after the next election. It depends on the popular vote and a whole range of factors and sitting down, as I'm willing to do with you, after the next election," Mr Xenophon said.

Mr Xenophon rejected suggestions his party was going easy on the Government, and flagged the announcement of more SA Best candidates in Labor-held electorates.

The fledgling party is set to unveil another handful of candidates today, including Port Adelaide-Enfield Mayor Gary Johanson, who will run against Education Minister Susan Close in the seat of Port Adelaide.

That prompted the Premier to accuse SA Best of splitting the conservative vote in South Australia.

"We've got another Liberal candidate running for the Nick Xenophon team," Mr Weatherill said.

"What's the explanation for the fact that so many of your candidates are former members of the Liberal Party?" he asked Mr Xenophon.

Weatherill sorry for Oakden, Families SA failures

The Premier was asked by Mr Xenophon to identify Labor's biggest failures during 16 years of government.

"The most appalling things have been when we haven't been able to protect vulnerable people," Mr Weatherill responded.

"So whether its aged care homes in Oakden, or little children in the state run facilities where we've had predators who've abused them.

"They're shocking failures. They can't be papered over. And they're a source of enormous distress for the people who are affected, but also for all of us in positions of authority.

"They are awful things and I'm deeply sorry they've occurred on our watch."