First rule of elections is that the undecideds break 100% for the Republican Party, so Clinton is actually trailing everyone except Huckabee.
It depends upon the state. In general, I have largely seen that the undecided break ineffectively toward the loser. If the split is 64-20, even a 100% break among the undecided toward the eventual loser still indicates a landslide. Also, if a nominee has a ceiling far short of 50%, then a 42-41 lead can become a 56-44 loss (look at Obama in the Dakotas in 2008).
Michigan is the sort of state in which the Republican nominee often has a ceiling around 45%.
So normally I expect 10%+ leads to shrink.
