And, in the month since, change has been at the heart of his every policy announcement, debate appearance and media engagement.

Marshall has promised a change in transport and health spending, in shopping hours, council rates and energy prices. He's fronted debates on social issues, economic issues and environmental issues, and, each time, he's promised change.

It's familiar territory for a man who has spent his entire political career in opposition.

Momentum during the 2018 campaign has waxed and waned between Marshall, Premier Jay Weatherill and newcomer Nick Xenophon, leader of SA-BEST.

The release of the Independent Commissioner Against Corruption's report into Adelaide's scandal-plagued Oakden nursing home came two weeks in and hit Labor hard.

Marshall capitalised. The Liberals followed the report's release with a string of announcements calling on the premier to resign. They were also promising new mental health facilities, to cut overdue elective surgery waiting lists and to extend palliative care.

In what could have been a sign Marshall was feeling optimistic about his public support, he cast his own vote a week and a half early.

But he was forced to issue an apology just days out from the poll, after the state's Electoral Commission branded the savings contained in the Liberals' energy plan as "inaccurate".

Marshall was also dogged by questions over a rumoured $1.2 million donation to the Liberal Party from Chinese businesswoman Sally Zou.

However successful the Liberal campaign has been, the introduction of a third major party into the mix has made the 2018 election near impossible to call.

It is hard to know whether Xenophon's SA-BEST will pull more voters away from the Liberal or Labor parties, and from which seats those votes will be pulled.

Whatever happens, Marshall has ensured state politics will look very different come the end of March.

He has declared he will either form a majority government, bringing an end to 16 years of Labor rule, or he'll resign from the leadership.

Marshall won the seat of Norwood (now Dunstan) for the Liberal Party at the 2010 election, and moved to the opposition's front bench the following year.

He became deputy leader of the party in 2012, and took on the leadership in 2013.

At the 2014 election, the stage looked set for Marshall to put an end to Labor's then 12 years in government and become the premier of South Australia - but his seemingly meteoric rise was brought to a halt.

The Liberals won 53 per cent of the two-party preferred vote with a swing of 1.4 per cent, though not in the right places.

The party picked up three seats from Labor and one from an independent but still fell short of the six it needed to form a majority government.

After several days of negotiations, Labor was returned to power when independent Geoff Brock agreed to give his support to form a minority administration.

It must have been a crushing blow for Marshall, and certainly raised doubts about his leadership capability, yet four years later he remains at the helm.

This time around, Marshall has a chance to make his mark in South Australian political history as the man who put an end to the Labor era - or he could return to the back bench and join the list who have tried.

Australian Associated Press