Nick Clegg's interview in the Sunday Times and on the BBC here warns Labour that having the third lowest number of votes undermines their mandate, irrespective of the number of seats.
Yes, the electoral system is potty, as Clegg says, and the polls are suggesting the Tories losing votes to the Liberal Democrats in marginals in the South. This could lead to 100 plus seats for Clegg's party, but with Labour having the largest number of seats on the third largest size of the vote.
Technically, a hung parliament would see the party with the largest number of MPs talking to Clegg. His shift of emphasis now rather points to the party with the most votes having the stronger mandate from the electorate. In short, it means that every vote counts.
I do find it slightly irritating round here that the LibDems say Labour can't win. It may be historically true, but tactical voting undermines and demeans people's right to state their choice.