How much of a swing in the electorate would the opposition need
to dethrone the PAP in 2015?
In a very accessible article, Jeraldine Phneah writes about the advantages the People's Action Party has over the opposition in Singapore. Note that several of the advantages are institutional and arise from the PAP's position as the dominant party in Singapore politics.
In PolSci speak, Singapore belongs to a subset of democracies that are called dominant party states: these are countries whose political landscape are overwhelmingly dominated by a single ruling party, often for decades. As it turns out, a phenomenon like the PAP is not that unique to history or politics. And as it turns out, phenomena like the PAP do come and go.