The first national poll conducted after Barack Obama publicly acknowledged Joe Biden as his running mate shows that Obama and McCain are virtually tied in this presidential race.

According to a new CNN/Opinion Research Corp. poll released Sunday night, 47 percent of those questioned are supporting Obama, with an equal amount backing his 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."

"It's not that there's anything wrong with the choice of Joe Biden. A majority rate the Biden selection as excellent or pretty good. Voters think he is qualified to be president, and with the exception of Al Gore in 1992, the public ranks Biden as the most qualified running mate in recent times," Holland said.

"A lot of Americans don't know who he is, but his favorable rating is 13 points higher than his unfavorables. But Biden is not Hillary Clinton, and it's possible that is enough to have moved some of her supporters away from the Democratic ticket, at least temporarily," opined Holland.