Four are confirmed — McCain, Huckabee, Tommy Thompson, and America’s Greatest Hero — and campaign sources think the rest of the field could go this week. Whom to thank?

Some of the campaigns have said the originally targeted September 17 date conflicted with their fundraising schedules. Ms. VanSickle was not able to confirm which campaigns have officially agreed to participate or when an official announcement might come.

But the campaign of former New York Mayor Rudolph W. Giuliani, which privately questioned the debate format, has made a number of public statements indicating he is open to participating. Former Massachusetts Gov. Mitt Romney appears less convinced after being the target of several YouTube videos questioning his credibility on issues of importance to social conservatives.

Former Govs. Mike Huckabee of Arkansas and Tommy G. Thompson of Wisconsin are on board.

Multiple sources close to the discussions say it was pressure from conservative bloggers, not scheduling conflicts, that made the reluctant Republican candidates reconsider.

It’s unfair but if Mitt ends up the lone holdout he’s going to face another round of religious demagoguery for it, e.g. “Mormons can’t deal with the ‘edginess’ of YouTube or young America.” I think he’ll show; in fact, I thought the whole field would show just because of the prisoner’s dilemma inherent in the fact that McCain had already committed and Fred would be likely to once he’s in. Rudy and Mitt can’t afford to cede that stage, particularly given the ratings of the last YouTube debate.

As for the influence of righty bloggers, I think this is/was a special case due to the lead role Patrick Ruffini and David All took on it. Both are well respected new media consultants in GOP circles; if it had been a simple case of me and the usual crew howling into the void, with no support from talk radio, we’d have eaten it. Which is to say, it’s not really “bloggers” so much as “Republican advisors who blog” who deserve the credit.