And it comes after Joe Biden’s addition to the Democratic ticket, too. CNN’s latest polling shows that Barack Obama has lost a seven-point lead since July — and that’s among adults, not registered voters or likely voters.  What happened?

In a new CNN/Opinion Research Corp. poll released Sunday night, 47 percent of those questioned are backing Obama, the presumptive Democratic presidential nominees, with an equal amount supporting his Republican opponent, McCain.

“This looks like a step backward for Obama, who had a 51 to 44 percent advantage last month,” said CNN Polling Director Keating Holland. “Even last week, just before his choice of Joe Biden as his running mate became known, most polls tended to show Obama with a single-digit advantage over McCain.” …

Sixty-six percent of Clinton supporters — registered Democrats who want Clinton as the nominee — are now backing Obama. That’s down from 75 percent in the end of June. Twenty-seven percent of them now say they’ll support McCain, up from 16 percent in late June.

The final choice of Biden seems to have dispirited the Clinton die-hards.  CNN’s internals say that Biden himself isn’t the issue, although he’s hardly generating enthusiasm, either.  Instead, Obama’s selection means that they have to face that Hillary won’t be on the ticket in any capacity.  The fact that Obama never even bothered to vet Hillary doesn’t help, either.

Now every major poll shows Obama losing his lead, and at least one — Zogby — shows McCain leading.  Zogby polled likely voters, though, which CNN did not.  They polled “adults”, normally the most sympathetic sampling for Democrats.  If Obama can’t show a lead with this sample, the Zogby poll looks less like an outlier and more like an accurate diagnosis of how voters see the race.

Obama needs a big bounce from the convention, but he only has hours to generate it.  John McCain will announce his running-mate the day after Obama’s acceptance speech and will own the weekend news cycle, especially since he’s very unlikely to botch that rollout as Obama did.  The Democrats find themselves in a tight corner, and with fewer allies every day.