The Biden Bump…

...went the wrong way.

The first national poll conducted after Barack Obama publicly named Joe Biden as his running mate suggests that the battle for the presidency between the Illinois senator and John McCain is all tied up.

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.”

So what’s the difference now?

It may be supporters of Hillary Clinton, who still would prefer the New York senator and former first lady as the Democratic Party’s presidential nominee.  Watch are Clinton backers on board? »

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 number of Clinton Democrats who say they would vote for McCain has gone up 11 points since June, enough to account for most, although not all, of the support McCain has gained in that time,” Holland said.