Barack Obama has had an unprecedented slide in polling over the summer, losing as much as twelve points nationally and squandering momentum gained from besting Hillary Clinton in the primaries.  Gallup’s polling has mirrored that of almost every other national survey, and they drill down to find the answer to Obama’s decline:

Barack Obama has been struggling to maintain his Democratic base thus far in August, and according to weekly averages of Gallup Poll Daily tracking, the problem seems to be with conservative Democrats.

Within the Democratic Party, Obama’s losses are primarily evident among the relatively small group that describes its political views as conservative. The 63% of conservative Democrats supporting Obama over McCain in Aug. 18-24 polling is the lowest Obama has earned since he clinched the Democratic nomination in June. At the same time, there have been no similar drops in support for Obama in the preferences of liberal or moderate Democrats.

As a result of this, support for Obama among all Democratic registered voters fell from 81% in early August (Aug. 4-10) to 78% last week (Aug. 18-24). Obama’s support from Republicans over this period also dipped from 9% to 7%, while 42% to 43% of independents have consistently supported him.

It’s not just conservative Democrats, either, although that has to be Obama’s main concern at the moment.  Blue-dog Democrats have to run for tough Congressional races in the fall, and they will get linked to Obama regardless of whether they publicly embrace him or not.  If Obama is losing credibility among conservative Democrats, the Blue Dogs will find that Obama’s impact on their race will become a net negative and could cost some of them their seats — most of which they won from Republicans in 2006.

Obama also have troubles with moderate and liberal Republicans.  At the beginning of the summer, 20% of these voters supported Obama, according to Gallup.  Now that has dropped to 13% and has dropped sharply from the time Obama traveled to Berlin.  In fact, the graph looks like a straight line from mid-July to today.

Married women have also begun rethinking their support for Obama.  He has lost seven points in two weeks in this demographic, and three points among unmarried women as well, although he still has a bare majority of the latter.

So who does this leave?  Liberal Democrats.  Obama has to hope that there are enough of those to carry him to victory, but as we can already see, that hasn’t kept him in the lead even during a summer when he should be trouncing McCain.  His strategies of tacking towards the center and flip-flopping on numerous issues hasn’t bought him any credibility with centrists and moderates, and in fact has undermined his campaign, perhaps fatally.  Democrats may have to work to contain the damage of an Obama collapse, instead of preparing for a presidential party in November.

Addendum: Allahpundit forwarded me today’s Rasmussen tracking poll, which includes surveys taken yesterday after Day 1 of the Democratic convention:

The Rasmussen Reports daily Presidential Tracking Poll for Wednesday shows Barack Obama and John McCain each attracting 44% of the vote for the second straight day. When “leaners” are included, though, McCain picked up another point since yesterday and now has a statistically insignificant one-point advantage over Obama, 47% to 46%.

This is the first time since August 9 that McCain has held any advantage over Obama. The candidates have been within two points of each other on every day but two for the past month.

The tracking poll calculates surveys over a rolling three-day period.  This poll would include data from Sunday, Monday, and Tuesday.  A gain by McCain today means that two events from which Obama would have hoped to garner a boost fizzled: the Joe Biden pick and Michelle Obama’s speech on Monday evening.