Sorry Grufty this allegory makes no sense to me
You say "Let's imagine that a post-election poll suggested that large numbers of would-be Labour voters in key Tory/Labour marginals were influenced by that campaign." I know better than to infer from your posts but hopefully we can agree that this influence would have manifested itself in people shifting their votes away from Labour. (or not voting at all) - unless you think it might have influenced them to vote for Labour.
You then propose "Except, even that does not demonstrate (or even imply) that the SNP are the cause of that vote transfer. It demonstrates that the tory PR / propaganda was successful." Somewhat splitting hairs, yes you could argue that the Tory PR was the cause but only because it was rooted in the truth of a strong SNP showing. I can't imagine that a 'Vote Labour, get the Orkney Independence Party' PR offensive would have yielded similar results.
You also say that "History is littered with bad campaigns that backfired badly." Its also littered with many that worked quite well 'Labour isn't working' and 'Yes we can' being just two that readily spring to mind?
Just to correct one final point - you're now talking about a "significant impact" south of the border. My initial post was made in reply to your assertion that "There is absolutely no evidence that the people of Bolton West shift their political allegiance based upon the perceived popularity of the SNP."? "Absolutely no".Indeed if you read back you'll see that in my first post I wrote "to answer your question the SNP (or the spectre thereof) may well have influenced a vote or two in Bolton West...." - note, a vote or two.
You can always 'win' an argument by posting a welterweight of website links - although a cursory Google search on 'people put off labour by fear of SNP' also throws up numerous pieces (allegedly back by polls) which could pass muster as evidence of people's votes being influenced by the 'vote Labour, get SNP' message.
Please don't conflate my posts with other peoples. I've never (nor would I have) claimed that the SNP were the reason Labour lost the election - indeed I think my posts made that clear. I've merely stated my view that the strength of the SNP in Scotland was a card seized upon by the Tories, that IMHO will have had some (negative) impact on the Labour vote south of the border.
Like I say its only an opinion - and not one that I thought was ridiculously contentious - I recall a news piece in the run up to the GE (filmed in Berwick upon Tweed IIRC) which had vox pops of people claiming to be Labour voters who wouldn't be voting for fear of putting the SNP "into power". Unsubstantiated, anecdotal, undoubtedly........beyond possibility....I don't think so
