Obama, D-Ill., arrived in Germany Thursday to launch the European half of his trip with the strong expectation that Obamania would be far stronger in Germany, France and Britain than it was even in the Middle East or, for that matter, at home in the United States. Meanwhile, most U.S. polls still give him a sturdy though not overwhelming lead over Sen. John McCain, R-Ariz., the GOP's 70-something presumptive nominee. However, a lead of 47 percent to 41 percent, which the NBC/Wall Street Journal reported in its latest survey published Wednesday, is still far from overwhelming.

How, then, to reconcile the striking and apparently even blessed success of Obama's progress through the war-torn nations of Afghanistan, Iraq, Israel and the Palestinian territories with his failure to generate any significant "bounce" from this in domestic U.S. popular support, where it really matters?

The answer appears to be rooted in America's old politics of confrontation and divisiveness between two nations separated by geography, culture and emotion far more than by education and class.

Obama really hasn't transcended, eliminated or even narrowed that great chasm at all. The political and cultural Grand Canyon in the minds of the American people is as strong as it has ever been over the past 40 years.

People had made up their minds about Obama and McCain well before Obama launched his high-profile gamble to tour the Middle East and Europe and boost his foreign policy and security credentials. The young senator from Illinois has clearly succeeded dramatically in doing what he set out to, at least in the first half of his trip, which appeared to be by far the most difficult part of it. But his success has not jogged the polls significantly so far.

Clearly, Obama's international Grand Tour hasn't been the knockout punch his strategists hoped it would be. But it hasn't been the self-inflicted disaster his opponents hoped for, either. The defining events of the 2008 race for the presidency of the United States have yet to occur.

Yet such crucial turning points do happen, more often than not.

Incumbent Vice President Al Gore was trailing Texas Gov. George W. Bush by as much as 15 percentage points in national polls in the summer of 2000, and he looked set to be swamped in a landslide -- until he picked Sen. Joseph Lieberman, D-Conn., as his running mate.

In truth, Lieberman proved to be an awful campaigner: He was well-meaning, well-liked and respected, decent and nice, but he was also one of the most boring national campaigners of modern times. Even future Vice President Dick Cheney looked animated and dynamic next to him in their vice presidential candidates' debate.

Yet the choice of Lieberman, the first practicing religious Jew to be picked for a national presidential ticket by either of the main U.S. political parties, re-energized the apparently doomed Gore campaign, wiped out Bush's commanding lead and left the race a nail-biter. It stayed that way right down to the wire.

There was also a key time of change in the 2004 presidential campaign. Democratic nominee Sen. John Kerry, D-Mass., came out of his nominating convention in Boston in July that year within striking distance of incumbent President Bush in the polls. But the Bush campaign then took Kerry apart in an intense media blitz that discredited his combat record in Vietnam.

Kerry didn't have a clue how to fight back. He lost 6 points in the crucial Midwest swing state of Ohio alone in a single week in August, and after that he was never seriously in the running again.

It may well be too early for such a defining moment in the 2008 campaign. Neither major contender has yet chosen his running mate. And neither has yet presided over his nominating convention.

The recurring pattern of U.S. presidential races in recent decades has been that the crucial defining moments, or twists and turns, in the campaign come during the summer or right after the conventions. Insofar as recent history is any guide, whatever way the race looks in early September is the way it will look when voters finally bring closure in the first week of November two months later. The first half of September, therefore, is usually the decisive period when the race crystallizes.

Judged from this perspective, it may be disappointing to Obama that he hasn't gotten more of a bounce out of his Middle East and Central Asian odyssey. McCain is certainly still in contention, however much the Obama-worshiping mainstream U.S. media claim that he isn't.

But Obama still so far has pulled off a remarkable and significant political triumph on his current trip. He shines at carefully scripted set-pieces, just as his hero President John F. Kennedy did. Yet in 1960, Kennedy, for all his handsomeness, glamour and dazzling rhetorical skill, only scraped into the White House against the unlovable Richard Nixon in one of the closest races on record. Obama may have to be content with a similar outcome.