Howard Wolfson makes that argument to ABC News, and points to Iowa as evidence.  Barack Obama put himself firmly on the path to the nomination by scoring a mildly surprising win in the first contest of the primary, while Hillary stumbled to a third-place showing.  If Edwards hadn’t been a factor, the Clinton campaign adviser believes Hillary would have won, and Obama would never have grabbed the momentum:

Sen. Hillary Clinton would be the Democratic presidential nominee if John Edwards had been caught in his lie about an extramarital affair and forced out of the race last year, insists a top Clinton campaign aide, making a charge that could exacerbate previously existing tensions between the camps of Clinton and Sen. Barack Obama.

“I believe we would have won Iowa, and Clinton today would therefore have been the nominee,” former Clinton Communications Director Howard Wolfson told ABC News.com.

Clinton finished third in the Iowa caucuses barely behind Edwards in second place and Obama in first. The momentum of the insurgent Obama camaign beating two better-known candidates — not to mention an African-american winning in sucn an overwhelmingly white state — changed the dynamics of the race forever.

The National Enquirer made the story public in December, after weeks of handwringing by the Los Angeles Times, which sat on the story.  Had the media taken the bait then (December 19th), Edwards could well have lost a substantial amount of support in Iowa two weeks later.  However, what the Enquirer lacked in December was smoking-gun evidence, which it got a few weeks ago when their photographers caught Edwards skulking out of Rielle Hunter’s hotel room in the middle of the night.  Without that evidence, the media had no reason to even question Edwards based on an anonymously-sourced rumor in a less-than-reliable celebrity gossip rag.

Let’s not forget, too, that the Enquirer has its own indirect ties to the political campaign, or did at the time.  Ron Burkle of Yucaipa Companies had worked for months to buy the Enquirer’s parent, AMI.  Bill Clinton also worked with Ron Burkle at Yucaipa as his senior adviser, and made a lot of money with Yucaipa’s investment funds.  The week before the Enquirer story appeared, Bill Clinton suddenly announced that he would “curtail his business relationship” with Burkle.  With those kind of indirect ties, the media would certainly have reason to consider their editorial choices very, very carefully in regards to unsubstantiated gossip.

Wolfson can blame Edwards if he wants, but this nomination was Hillary’s to lose — and that’s exactly what she did.  In November of last year, she made an unbelievable flip-flop on illegal-alien drivers licenses on national TV within 120 seconds, which set Hillary off balance for weeks.  She began criticizing Barack Obama over his kindergarten essays, which made her look ridiculous and petty, and Obama exploited the opening to undermine her credibility to be presidential.

This sounds like a whole lot of blame-throwing — which, come to think of it, really is vintage Clinton.

Here’s the magic moment that started the downhill slide: