Whatever I am going to say here is purely unscientific and is based on a gut feeling resulting from observing current debates about the Presidential elections.
From conversations with my friends and observing online chatter it is clear that all the usual anti-PAP crowd is anti-Tony Tan. I believe that there is no doubt that based on the GE 2011, the same 40% that voted for the opposition would not be voting for Tony Tan. But, the big question is whether they would all instinctively gravitate towards a single candidate. Tan Cheng Bock, Tan Jee Say and Tan Kin Lian all have their merits and attractions for the anti-PAP voter.
What I find pretty clear as well is that a sizeable part of the 60% of PAP voters would not be voting for Tony Tan who is popularly perceived to be the PAP endorsed candidate. I know of friends and relatives that voted for the PAP in the GE for several reasons that would play no part in the presidential elections:
a) constituency level issues
b) fear of losing a PAP government
c) fear of losing credible Cabinet ministers
d) fear that they would lose out perks or promotions because they are in the civil service
e) fear that they will be 'marked' because they voted against PAP
I am sure that about 30% of voters would have been hardcore PAP supporters. These individuals would vote for the PAP endorsed candidate.
(I have always believed that the PAP has about 30% of hardcore supporters and there are about 25% to 30% hardcore opposition supporters. The remaining 40% to 45% of voters could be persuaded).
Nearly half of the voters that voted for PAP are likely to vote for someone other than Tony Tan. It is easy to assume that the person who benefits from this would be Dr Tan Cheng Bock given that he was a PAP MP and the PAP voter that decides not to vote for Tony Tan may not be inclined to vote for either Tan Jee Say or Tan Kin Lian.
The pro-opposition camp appears to me to be leaning towards Tan Jee Say. But, the 40% of opposition voters are not going to naturally veer towards Tan Jee Say and Tan Kin Lian. Some of my friends have expressed their preference for Dr Tan Cheng Bock. Though he was a PAP MP in the past, he has enough of a track record in opposing the official policy position of his Party. If elected, we can expect him to be a President like Mr Ong Teng Chiong. Given this dynamics, I wouldn't be surprised if the 40% contribute a little to Dr Tan Cheng Bock's votes.
Dr Tan Cheng Bock might turn out to be the biggest beneficiary of a 4-cornered fight picking up votes from both the opposition as well as PAP voters. He also has the requisite personality to endear himself with the people.
Putting aside 30% of the votes for Tony Tan, the remaining 70% would probably be split amongst the Three Tans. There is a good chance that TCB can garner 35% of the votes with TJS and TKL picking up the balance 35% between the two of them.
If on nomination day all 4 Tans contest, Dr TCB stands the best chance of winning.
My own hope is that the Three Tans come to a consensus and two of them give way so that it would be a straight contest between TT and another Tan. The result would be clear. The next President would not be TT. But, with the three Tans going against TT, the split in the votes could be such that TT might get in by the narrowest margin by getting as little as 30% because the other three could get 25%, 25% and 20% That would be a result that most of us would not want. I hope it doesn't come to pass that way.