I agree with whoever said the only way this is even slightly plausible is if their is a succesful hardliner coo in the USSR in 1991 and eventually a major Protect and Survive style crisis that builds in the run-up to May 1st 1997 (or whenever the election happens). Obviously this crisis can only gain a certain amount of momentum-so basically the balance is dificult.
Perhaps someone in the labour camp makes their polecy seem weak on the resurgent Soviets, in comparrison to the Tories, while someone else from Labour severely criticises the government for their handling of said crisis, leading to people thinking that Labour are trying to gain political capital out of something which should be handled in a bipartisan manner.
So basically, Labour have to be collectively stupid and things have to play out in a certain way to make this a possibility. Even then, you could argue that if an election was going to be decided on that basis in the first place, then there would be a likelyhood of war approaching-and there probably wouldn't be an election as a result-more likely you'd end up with a National Government or something.
So yeah, this is almost ASB.