Here’s the graph of polling error against probability of victory we saw before, updated for the latest polling data:
So if you assume that the polls will be out by 5% or less, then Obama’s chances of victory against McCain are over 60%, but they dip under 60% when you assume a larger polling error.
However, that accounts only for random error in the polls; we assume that each state’s final tally will differ from today’s polls by a normally distributed value which is totally independent in each state. In reality, there is likely to be a systematic, country-wide change in his standing against McCain between now and voting.
Suppose he gains one percentage point countrywide. We can simulate that by just adding 1% to his margin in each poll and re-running the test:
With 1% extra his chance of victory is now between 60% and 80%, depending on how big you think the random error is. Now let’s take off 1%:
and we see that if he loses 1% he has less than a 50% chance of winning the election. Finally let’s bracket that with estimates for +2% and -2%:
Here’s the same picture, going by Clinton’s poll results against McCain:
which shows that if we assume a 5% error in the polls (a rather low estimate) then Clinton can lose 2% of her popularity nationally and still be virtually guaranteed victory.
Here are the two superimposed:
We see that if we assume a 10% error in the polls, then Clinton can lose 2% of her national popularity and still be ahead of where Obama is now.
No-one is talking about Clinton’s apparent huge advantage over Obama in the Electoral College. Is there some systematic error in the polling? Are Rush Limbaugh fans claiming to be Clinton supporters, as per his encouragement, to throw the Democrats into confusion? Or should the few remaining uncommited supers be throwing themselves behind the losing nomination candidate in a desperate bid to keep the one who will guarantee them victory in November?