OUTGOING Labor senator Don Farrell has abandoned his bid to enter South Australia's parliament hours after Premier Jay Weatherill threatened to resign in protest.

"Today, if there is any suggestion that there is doubt about who is actually running the show, it's been made absolutely clear that it's me," the premier said on Friday afternoon.

In a morning of extraordinary politics, Finance Minister Michael O'Brien announced he would stand down from his safe Adelaide seat of Napier to make way for right-faction powerbroker Senator Farrell before the March 15 state election.

He said he offered to stand aside for his good friend the day after the federal election when it was clear the senator would lose his seat.

The senator surrendered his No.1 Senate ticket position to Penny Wong during the protracted factional brawl over the federal Labor leadership.

Mr Weatherill then told ABC radio he would consider resigning if the senator was pre-selected for Napier.

The SA Labor Party had been a unified party, while "for better or worse" Senator Farrell was associated with the disunity that led to the destruction of the federal party at the election, he said.

"We don't want that in state parliament."

While Senator Farrell said he was not interested in the SA ALP leadership, the premier said he regarded the senator as a threat to him.

"It is my responsibility as party leader to present a unified team and the clearest-possible message to the electorate that you know who you are voting for when you vote at the next state election," he said.

Shortly after, Mr O'Brien and Senator Farrell fronted the media and the senator withdrew his nomination, saying he did not want the Labor Party to lose the election.

"On the ABC radio this morning, the premier made it very clear he does not wish me to contest this pre-selection," he said.

"I've always put the party first. I've been a party person all of my life.

"I don't want to see any division in the party over this issue so as a result I am withdrawing my candidacy bid."

The premier later described the move as a "factional try-on" designed to oust him.

"I was determined to repel any threat to that (SA Labor) unity and I made that very clear to anybody who proposed any matter to me," he said.

SA opposition health spokesman Rob Lucas said Senator Farrell had exposed the deep divisions and disunity in a dysfunctional SA Labor Party.

Any cabinet minister who supported the push for Senator Farrell for the Labor leadership must resign, he said.

The Liberal Party tweeted a photo of Mr Weatherill in front of Senator Farrell with former prime ministers Kevin Rudd and Julia Gillard and former premier Mike Rann in the background - and the caption "who's next?"

Mr O'Brien later offered his resignation and the premier said his portfolios had been allocated to existing ministers on an acting basis.

Mr Weatherill said he would take over the finance portfolio, Deputy Premier John Rau would take police and correctional services, and emergency services and road safety would go to Ian Hunter.