The South Australian Liberals are battling to retain the prized state seat of Bragg in a by-election forced by the resignation of former deputy premier Vickie Chapman.
With about 50 per cent of the vote counted on Saturday, Liberal candidate Jack Batty was clinging to a narrow lead over Labor's Alice Rolls after another big swing to the ALP.
Mr Batty had 51.1 per cent of the two-party preferred vote to 48.9 per cent for Ms Rolls.
In the March state election, Ms Chapman's 16 per cent margin was halved to eight per cent.
But even that has almost been wiped out in another strong performance by Labor, with the party also benefiting through preferences from a strong performance by the Greens.
Greens candidate Jim Bastiras had attracted almost 18 per cent of the primary vote, well up on the 13 per cent the party managed at the state election.
About 9000 voters cast either an early ballot or applied to vote by post in Bragg, with those ballots not counted until Monday. They could now prove crucial to the result.
Before the polls closed, Opposition Leader David Speirs said retaining Bragg would be the first of many stepping stones to return the Liberals to government in 2026.
He described the by-election as a crucial opportunity to road test the party's new direction with voters.
If the Liberals do hold Bragg, the opposition will have 16 MPs in parliament's 47-seat House of Assembly.
The Labor government holds power with 27 seats with the remaining four held by independents.
