The latest Zogby poll reinforces what earlier LA Times/Bloomberg and Pew polls have found: Barack Obama has had a collapse this summer.  Being Zogby, it has a more dramatic collapse than the others.  Obama led John McCain in Zogby’s July poll, 47%-40%, but now trails 46%-41%, a 15-point swing (via JWF):

As Russian tanks rolled into the Republic of Georgia and the presidential candidates met over the weekend in the first joint issues forum of the fall campaign, the latest polling includes drama almost as compelling – Republican John McCain has taken a five-point lead over Democrat Barack Obama in the race for President, the latest Reuters/Zogby telephone survey shows.

McCain leads Obama by a 46% to 41% margin.

And McCain not only enjoys a five-point edge in a two-way race against Obama, but also in a four-way contest including liberal independent candidate Ralph Nader and Libertarian Bob Barr, the poll reveals. In the four-way contest, McCain wins 44% support, Obama 39%, Barr 3% and Nader 2%.

This latest Reuters/Zogby poll is a dramatic reversal from the identical survey taken last month – in the July 9-13 Reuters/Zogby survey, Obama led McCain, 47% to 40%. In the four-way race last month, Obama held a 10-point lead over McCain.

One has to take Zogby with a grain of salt.  Over the years, the predictive value of Zogby’s polling has been questioned, especially this far out from an election.  Zogby has enjoyed some success, too, but usually their results tend toward the dramatic. As a trend, though, it shows the same motion as every other pollster.  Obama did well in July with Zogby, which was actually a bit of an outlier in estimating Obama’s support as strong as seven points head-to-head and 10 in a four-way race.  The same polling shows a dramatic change in the electorate, as does the LAT/Bloomberg and Pew polls.

Zogby’s internals show this change across almost every demographic, too.  Obama lost nine points among Democrats.  He lost 11 poinst among Catholics, college graduates, Southerners, and city-dwellers, where he has dropped below 50% — a stunning result.  Obama dropped 12 points among voters under 35, another power base for his campaign, and seven points among those earning under $50,000 a year.

The difference — and perhaps the reason for the drama — is that this poll surveyed likely voters, while Pew and the LAT/Bloomberg polls surveyed registered voters.  The latter usually favors Democrats, and that could account for the seven-point difference between the two.  The month-on-month results show trends that speak for themselves, as Zogby’s methodology has remained consistent.

Obama is in a free-fall.  McCain got to the heart of the question about Obama, and especially after the crisis in Georgia, he has voters wondering whether Obama is up to the task of running the presidency as his first executive job in politics.  Even a strong VP pick may not help reverse that perception — and could make it worse by overshadowing the nominee.  Democrats may not have been in a weaker position at convention time since 1984.