Somehow, the laws of political physics have changed.
Mickey Kaus's theories:
1. "[L]ate-focusing voters tune in to what the press is saying in, say, the two days before their state's election, which is usually something different from what the press says in the two days after the previous state's election."
2. "[V]oter rebellion against what the press says--Huck's Hot! Barack Rock Star!--and it's overdetermined."
Surely, we can think up some more theories. I'll start:
1. As long as we think a particular candidate isn't going to win, we feel free to embrace him, but once we see him realistically as President, we get cold feet.
2. When a candidate is first emerging, he has more control over his image and can make us like him, but if he's the frontrunner, his opponents do what they can to make him look bad.