WITHOUT a doubt, Edinburgh South is one of the key battlegrounds in this election, if only for the fact that the constituency returned Scotland’s only Labour MP in 2015.

Ian Murray won the seat by just more than 2,500 votes from the SNP’s Neil Hay, who ruined his chances with some unfortunate messages on social media. When will politicians learn that Twitter and Facebook are not their friends?

Murray’s victory amid the tsunami of 56 SNP seats was not unexpected but he has never looked entirely comfortable with the role of the only Labour MP in Scotland, not least because he is very strongly against the renewal of Trident – he voted that way in the House of Commons last year – which was also Scottish Labour’s policy until their manifesto U-turn was published.

Now Murray faces a challenge from the SNP in what was for most of the 20th century a pretty safe Conservative/Unionist seat. Nigel Griffiths won it for Labour in 1987, and when Murray succeeded him in 2010 he had a majority of 316 over the LibDems, with the Conservatives in third and the SNP a poor fourth.

Murray would normally have had a huge advantage over the other candidates in that he has held the seat for seven years and, to be fair, has worked assiduously at building up his local support.

His SNP opponent, however, is local man Jim Eadie, who represented the area in the Scottish Parliament from 2011 to 2016. He lost out last year to Labour’s Daniel Johnson in what was that party’s only gain from the SNP.

The Conservatives have also gone with a well-known local candidate, newly elected city councillor Stephanie Smith, whose biggest problem is that Edinburgh South was one of the strongest areas for Remain in the Lothians in last year’s EU referendum.

The quartet of candidates is completed by Alan Beal of the LibDems who stood for the Colinton/Fairmilehead ward in the council elections of 2012 and came a poor sixth in the first preference vote.

That Edinburgh South seems to be a Labour-SNP contest is quite remarkable given that the constituency includes matronly Morningside and leafy Liberton, where the Conservatives used to rule the roost.

Gilmerton and Moredun would normally be seen as safe Labour territory, yet the ward returned two SNP councillors at the recent local elections, compared to one each for Labour and the Conservatives.

That Tory was Stephanie Smith, and the outcome of the election in this constituency may well depend on one matter – given the Conservative resurgence, do Tory voters back her when tactical voting to keep out the SNP would dictate a vote for Labour?

One statistic released at the weekend proves how much Labour want to retain this seat – our sister paper the Sunday Herald reported the leaked information from the Scottish Labour Party that Edinburgh South was the top constituency for "voter contacts", which measure the local party’s interaction with the community.

Yet the number of contacts recorded was just 2,578, down from 4,459 at the same stage in the 2015 campaign. All the same, Edinburgh South residents speak of seeing Labour activists “all over the place” and while the SNP are apparently keeping their powder dry for a late surge, the Conservative rosettes are back out. The bookmakers say the constituency is the most gambled on in Scotland and most of the money has been for Labour, which is why Murray is odds-on favourite to win.

Much will depend on where the “lost” votes go. For in common with 56 other constituencies, Edinburgh South will have no Green Party candidate, and two other parties are not repeating their candidacy of 2015.

Last time, the Greens’ Phyl Meyer polled 2,090 votes and Ukip's Paul Marshall got 601, with the Colin Fox of the Scottish Socialists polling 197.

None of those parties are standing, and both the SNP and Labour are making a strong pitch for the Green vote.

Reflecting the fact that no-one can really predict the result yet, at Cameron Toll shopping centre beside Inch Park at the weekend, voters were being canny about their choices.

Andy McIntyre, a National reader, said: “For many people the two issues here are nothing to do with local matters and everything to do with Westminster and the Tories’ attitude to a hard Brexit and to Trident.

“Ian Murray has a lot of local support but Scottish Labour’s decision to back a renewal of Trident is going to hurt him. He has also just kept parroting Kezia Dugdale’s rants against a second independence referendum, but a lot of Labour voters feel betrayed by the Unionist parties taking us out of Europe.” Catherine Gillespie said she was not sure who to vote for, but Murray had helped her local community centre so she would probably vote for him.

In the nearby bookmaker’s shop, a punter would only say that politics wasn’t worth betting on “because these days, nobody can guess how it will turn out.”

So very true on a global scale, and certainly that is pretty much the case in Edinburgh South, where the result is likely too close to call.