About 70,000 homes in the north-east are being sent leaflets outlining ways to vote tactically to derail the SNP surge and Alex Salmond’s return to Westminster.
Former Better Together campaigners in the region have organised the mail drop in three seats – Gordon, West Aberdeenshire and Kincardine, and Banff and Buchan.
The move comes as polls continue to show that the SNP is poised to make sweeping gains across Scotland at the election on May 7.
The Lib Dems claim only their candidate Christine Jardine can stop Mr Salmond in Gordon, while the Tories believe they are best placed to challenge the SNP in West Aberdeenshire.
About 30,000 homes in the Gordon constituency are due to receive the leaflets this week, with a similar number expected to be sent out in West Aberdeenshire and Kincardine, as well as several thousand in Banff and Buchan.
Campaigners in other areas of Scotland are believed to be doing the same.
Ian Lakin, who was chairman of the pro-UK Better Together campaign in Aberdeenshire during last year’s referendum, said: “This was separate from the parties. This was purely grass-roots people who were active in the Better Together campaign.
“If there had been no talk about another referendum from Alex Salmond and Nicola Sturgeon then we probably wouldn’t have bothered. It’s only because of that we’re doing it.
“We’re not telling people to do it, we’re just saying be aware of the choices.”
A senior Lib Dem strategist said: “This election is the most important chapter in Scotland’s history. If the unionist vote splits, delivering scores of SNP MPs, we should be in no doubt that the end result will be a second referendum.
“This is serious and in each of our 11 held seats, well known MPs like Charles Kennedy and Sir Robert Smith are gaining support from all parties to stop the SNP.”
A Conservative Party spokesman said: “Our message in this campaign is that people should vote for what you believe in. It’s simple – if you believe in keeping the economy strong and in keeping Scotland within the UK, then vote Scottish Conservative.”
North-east SNP MSP Christian Allard said it was “extraordinary” to see Labour and Lib Dem politicians backing attempts to elect more Conservative MPs in Scotland.
“The fact that tactical voting campaigners are actually urging a Tory vote in four seats that the Lib Dems currently hold and in one Labour seat shows that this is a pro-Tory front to try and get David Cameron back into government,” he said.
“That is the last thing that people in Scotland want, which is why the SNP has vowed that we will never do a deal to let the Tories back into office.”