Has there been another case of a major party imploding this badly before an election, save Greece? I know there's a few there but that's special circumstances.
Well, in Spain there is the infamous case of UCD in 1982 (much worse than National and on par with PASOK), though that was more of a gradual collapse throughout their entire 3 year term as the party gradually dissolved in a spiral of infighting. Though looking at the super old polling in Wikipedia it seems the collapse accelerated quite a bit in early 1982.