It wasn't any particular rough spot in the road that did it. Not the Jeremiah Wright thing. Not his vague association with Bill Ayers, an unrepentant terrorist from the violent '60s who's now, of course, a Distinguished Professor of Education. (Same ideology, different tactic. If you can't destroy the system with bombs, undermine the next generation.)
As for Barack Obama's suspect dealings with one Tony Rezko - restaurateur, political fund-raiser, land developer and general wheeler-dealer who's now under indictment - that connection doesn't seem to have made much of an impression, either. (Hey, it's Chicago.)
What's diminished the once bright young star of American politics has been the slow, gradual realization that, however elegant and appealing his manner, and it's both, he is the greenest of U.S. senators. Not that there's anything wrong with being young; his youth is attractive. But to be young and inexperienced; that's not assuring in a prospective president and commander-in-chief.
And the inexperience keeps showing, especially in foreign and military affairs:
He speaks as if setting an arbitrary deadline for American withdrawal from/surrender in Iraq wouldn't risk calamitous consequences.
In one breath, he criticizes Jimmy Carter for opening negotiations with a terrorist outfit like Hamas, and in the next he comes out for negotiating with Hamas' puppetmasters in Syria and Iran without preconditions - before backing away from that early incautious stand. Although not nearly far enough.
It's all enough to make him sound not only naive but indecisive about it. More and more, he comes across as less the bright new hope than the ordinary backing-and-filling pol.
One result of Hillary Clinton's unending tenacity in this race, even if she fails to win her party's presidential nomination, is that she'll have made the choice in November appear less between the new and old than between the vacuous and the authentic.
No wonder so many Republicans are rooting for her at this contentious stage. So far she's been a lot more effective than John McCain at revealing Barack Obama's weaknesses.