John McCain has erased a seven-point deficit in the latest CNN/Opinion Research poll and is now tied with Barack Obama at 47%-47%. Just three weeks ago, Obama led McCain 51%-44%.

Perhaps more significantly, the poll finds that 27% of Hillary Clinton supporters say they will vote for John McCain:

It’s a dead heat in the race for the White House. The first national poll conducted entirely after Barack Obama publicly named Joe Biden as his running mate suggests that battle for the presidency between the Illinois senator and Republican rival John McCain is all tied up.

In a new CNN/Opinion Research Corporation poll out Sunday night, 47 percent of those questioned are backing Obama with an equal amount supporting the Arizona senator.

“This looks like a step backward for Obama, who had a 51 to 44 percent advantage last month,” says 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,” adds Holland.

So what’s the difference now?

It may be supporters of Hillary Clinton, who still would prefer the Senator from New York as the Democratic Party’s presidential nominee.

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,” says Holland.

Clinton and Obama battled throughout the primary season, with Clinton winning more than 40 percent of the delegates. She suspended her bid for the White House and backed Obama in early June, after the end of the primary season.

A majority of registered voters, 54 percent, think Obama’s choice of Delaware Senator Joe Biden as his running mate is an excellent or good decision. That number jumps to 73 percent when just asked of registered Democrats. But it drops to 59 percent when narrowed to Clinton supporters.

“It's not that there's anything wrong with the choice of Joe Biden. A majority of those polled 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," says Holland.

"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."