What I'm looking for is a major party realignment in terms of regional or possibly class allegiance, like the switch of the American South from a Democrat to a Republican stronghold, in one of the bigger European countries.
In Europe, there are regions which would still in majority vote for a respective party even if a fox was their leading candidate: for instance, the industrial North-Western regions of England have been voting Labour for about 100 years, the South German states are Christian Democrat core territory. How could a major realignment be done? An interesting development would also to make descendants of immigrants - so far, considered a vote bank for centre-left/social democratic party voters - switch to conservative centre-right parties.
One could argue that Marine Le Pen's Front National is pursuing such a shift right now, as she's making inroads in former working-class Socialist areas.