A giant billboard poster is wrestled into a shop which contains little else but rows and rows of bright yellow pens, badges and flags emblazoned with the SNP logo.

In the newly opened campaign hub in Crieff High Street Donald Elder, 68, who has supported the SNP for nearly 50 years is confident his party's candidate Tasmina Ahmed-Sheikh will be elected to the Ochil and South Perthshire seat in just over three weeks time. "She will win it no bother," he says.

With a string of polls indicating a potential near-wipe out of Labour by the SNP, his optimism is not surprising. However in this constituency, which covers the Clackmannanshire Council and the southern part of Perthshire, is a marginal seat where efforts are underway by some activists to promote tactical voting. Widely seen as a contest between Ahmed-Sheikh and Labour's Gordon Banks, who has been MP for the area since 2005, they want to see Conservative and Liberal Democrat supporters to do what might ordinarily seem unthinkable - use their vote to back Labour in order to keep the SNP out.

It is an issue that Banks claims people are raising on the doorstep. On the campaign trail in the Ten Acres estate in Sauchie with a team volunteers carrying out the relentless task of knocking doors and delivering leaflets, he says: "We want to have the opportunity to convince everyone to vote for me in this constituency, but people know very clearly that this time round the constituency is a two-horse race between me and the SNP.

"People are raising that issue themselves and indicating that they are going to vote accordingly. They are voting outwith their normal comfort zone and their normal party allegiances."

He adds: "Tactical voting is not something new, it has happened since there has been voting."

Some analysts have suggested that tactical voting could make a difference. Last week a You Gov poll found the SNP is enjoying a 24 per cent lead over Labour - but when voters were asked if they would switch their votes to another party to defeat the SNP, the lead fell to 15 per cent. An analysis carried out by Channel 4 suggests this pattern could save Labour up to nine seats.

Banks is cautious when asked if he is confident of keeping his seat, saying that assumptions can never be made in elections. When asked about the SNP surging ahead in the polls, he contests that a battle for the constituency is nothing new.

"In 2005, the SNP were expected to win. I beat Annabelle Ewing by just over 600 votes," he says. "It was their number one target seat to win and they didn't win it.

"In 2010, my majority went up to over 5,000 in the same boundary against the same candidate. Again it was the SNP's number one target seat.

"Going into 2015 I don't expect it to be anything other than the SNP's number one target seat and we are used to that. It is nothing new."

One campaigner who is encouraging votes for Banks is Victor Clements - even though he is a Liberal Democrat member who doesn't even live in the constituency. Clements is a board member of the group Forward Together, which has been set up to encourage tactical voting in Perth and Kinross, including the Ochil and South Perthshire constituency.

Clements says the idea for setting up of the cross-party pro-union group - which mostly involves former Better Together activists - resulted from people regularly asking who was the right person to vote for in the 2015 election.

"We had the referendum last year and that was all over, we were glad to move away from that and leave it behind us," he said. "We all thought things were going to progress in a different direction now, but as time went on it became obvious it wasn't the case and a lot of people started asking us then who is the right person to vote for in this area next year.

"We came to the conclusion that if we were to try and influence the outcome of the election, what we would be asking people to do is to think about tactical voting."

The group is distributing 65,000 to 70,000 leaflets across two constituencies: Ochil and South Perthshire and also Perth and North Perthshire, where it is encouraging votes for Conservative candidate Alexander Stewart against the SNP's Pete Wishart, the incumbent MP.

"We knew because we had two constituencies - one of which had a sitting Labour MP and one in which the Conservatives are the challenger, that we had a bit of balance," Clements says.

"We decided we would put together a fairly simple leaflet setting out the information people wanted to know and we wouldn't be too forceful about it."

Clements claims people are happy to think about voting tactically if they believe their action will be balanced out by someone else in another constituency doing the same and potentially voting for the party they would normally back. As a resident of Aberfeldy, he is planning to vote Conservative in Perth and North Perthshire, despite being a Liberal Democrat member.

"It feels a bit surreal, but it is not something I am uncomfortable with," he says. "As a party member, you think it through and at first you think I can't do that.

"So although it is unusual for me personally, I think of the greater good, where we will probably get more votes elsewhere. My conscience is fairly clean on that."

Ochil and South Perthshire covers vastly differing areas, from the genteel villages and suburbs of Perthshire - such as Auchterarder and Crieff - to the working class estates of Clackmannanshire, which once relied on traditional industries such as coalmining and textiles. The constituency boasts tourist attractions including the luxury five-star Gleneagles Hotel - but also has one of the highest child poverty rates in the country.

Out on the streets, it is less than clear if the idea of tactical voting is having any impact. One pensioner who is enjoying the sun in the square in Crieff says she usually votes Conservative, but is considering switching to Labour for this election. But she says it is not to keep the SNP out. "It is because hopefully Labour will not touch our pensions," she says. "I am worried about that with the Tories."

Andy, who declined to give his full name, from Dollar, is outside a shop in Alloa. He says he is one of the few who votes Conservative in the constituency. But while he is aware of the issue of tactical voting, he has not had many discussions with people over who they intend to back. "It depends on whether you are voting for yourself or what you think is the best for the country - and it's a difficult one," he says.

In a housing estate in the centre of Alloa, SNP candidate Ahmed-Sheikh and her team of campaigners are out knocking doors. She is cautious about the idea of the race for the seat being between the SNP and Labour.

"On the doorstep there are obviously parties performing better than others," she said. "I certainly wouldn't like to project who might perform well or who is performing best at any given time.

"This is an election and all we are focusing on is speaking to as many people in the constituency as possible, allowing them to air their views, put across what our proposals are for the constituency and encourage people we are the party to deliver where the others have failed."

Ahmed-Sheikh also insists the issue of tactical voting is not something she is encountering on the thousands of doorstep which she has visited as part of her bid to oust Labour from the seat.

"No-one has said to me they had been approached by anyone asking them to vote in a tactical manner," she says. "They are just looking for who can best deliver at this point and time for Scotland."

In fact, Ahmed-Sheikh says she believes people who once might have backed different parties in Scottish and Westminster elections - are now more prepared to support the SNP this May.

"People are very aware of how to vote differently in different elections," she says. "There have been people who have been saying we are going to give you a chance, we would like to see what you would do if you have increased presence down at Westminster I have heard that a few times now, which is very interesting.

"People are now seeing there is a great opportunity for us to do a lot of good work at Westminster."