After the 1979 election, in which Margaret Thatcher defeated Jim Callaghan, Lord Thorneycroft, Tory party treasurer at the time, claimed that the poster had ‘won the election for the Conservatives’.
Language notes
The slogan "Labour isn't working" can be interpreted in at least two ways. The most obvious meaning is that the Labour government's policies are not effective—unemployment stood at 1.4m before the 1979 election (though it's now approaching 2m!). The word "labour" could also refer to the workers who are not working because they are unemployed. The caption at the bottom of the original poster— "Britain's better off with the Conservatives"—can also be read in two ways: better off in the sense of "more wealthy" or just "in a better state".