It has been a long campaign - maybe that can be South Australian Liberal leader Steven Marshall's excuse for a final day slip-up.

Visiting a chocolate factory in Adelaide, he told reporters the state's voters should opt for Labor when they go to the ballot box.

Then, without even flinching, he continued his news conference.

Mr Marshall only realised his gaffe when there was a question from a reporter later in the news conference about his slip of the tongue.

Opinion polls have consistently given the Liberals an edge heading into Saturday's election, at which Labor is seeking a fourth consecutive term.

The Liberals will need to snatch six lower house seats and hold their others to form government after this weekend.

Mr Marshall and Labor Premier Jay Weatherill have been doing the election eve rounds of television and radio stations, putting their final pitch to voters.

South Australia has fixed, four-year parliamentary terms.

Labor formed a minority government in 2002 and has been in power since, winning the 2006 and 2010 state polls.