Parties and pundits - both north and south of the border - are carefully watching the June 20 Aberdeen Donside Holyrood by-election amid signs that the outcome could be closer than expected.

The poll, a key pointer of public opinion on the path to next year's independence referendum, is also crucial to the SNP which would lose its overall Scottish Parliament majority if defeated in the constituency - next door to First Minister Alex Salmond's Aberdeenshire East stronghold.

At first glance this looks unlikely as Brian Adam, whose death caused the contest, had a 7,175 votes - 26.7% - lead over Labour in 2011. However a calculation based on the 2012 council polls suggests the fight is much closer.

Totalling up the votes cast in Donside's five Aberdeen City electoral areas gives a line-up of: SNP 8,490 (39.5%); Lab 7,367 (34.3%); Lib Dem 1,717 (8.0%); C 1,242 (5.8%); Others 2,680 (12.4%).

The SNP margin may be slightly boosted by the fact that only part of Hilton/Stockethill - which had a Labour lead - is in the constituency.

There have been more recent signs of further SNP slippage. Since the referendum agreement last year the party's vote share has fallen in five out of seven council by-elections in Scotland.

The main contenders are the SNP's Mark McDonald, who has given up his Scotland North East regional proportional representation seat, and Labour Aberdeen City councillor Willie Young.

Faced with a tight outcome between them, Liberal Democrats and Tories may find it hard to improve on their previous scores which were only just above the 5% lost deposit level.

Ukip has a candidate but the party traditionally polls less strongly in Scotland than in the rest of the UK.

The full line-up of runners is: Otto Inglis (Ukip), Christine Jardine (Lib Dem), Dave MacDonald (National Front), Mark McDonald (SNP), Tom Morrow (Scottish Christian Party), Rhonda Reekie (Green), Ross Thomson (Conservative), James Trolland (Scottish Democratic Alliance), Willie Young (Labour).

Aberdeen Donside 2011 Scottish Parliament election result: Adam (SNP) 14,790 (55.2%); Crockett (Lab) 7,615 (28.5%); Thomson (C) 2,166 (8.1%); McLeod (Lib Dem) 1,606 (6.0%); Henderson (Ind) 371 (1.4%); Willett (National Front) 213 (0.8%).