Here is why McCain decided to attend tonight's debate.

Thinkprogress:

Bailout negotiations “dissolved into a verbal brawl” at the White House yesterday, as some House Republicans, led by Eric Cantor (R-VA), said they wouldn’t back a bipartisan negotiation on the package. The House GOP faction stunned the participants at the meeting yesterday by announcing their own plan which “advocates tax cuts and relaxed regulations.”

Treasury Secretary Henry Paulson has said the House GOP proposal would not work. “Democratic leaders questioned McCain’s involvement in the House Republicans’ opposition to the plan.” McCain met with House GOP leaders before heading to the White House, but neither party seemed to know what they were talking about:

Boehner and McCain discussed the bailout plan, but Republican leadership aides described the conversation as somewhat surreal. Neither man was familiar with the details of the proposal being pressed by House conservatives, and up to the moment they departed for the White House yesterday afternoon, neither had seen any description beyond news reports.

At 1:25 p.m., McCain left Boehner’s office through a back door, walking across the Capitol’s rotunda to the applause of tourists. Graham conceded the group knew little about the plan the nominee had come to Washington to try to shape.

And there is more:

NYTimes analysis:
Senator John McCain had intended to ride back into Washington on Thursday as a leader who had put aside presidential politics to help broker a solution to the financial crisis. Instead he found himself in the midst of a remarkable partisan showdown, lacking a clear public message for how to bring it to an end.

At the bipartisan White House meeting that Mr. McCain had called for a day earlier, he sat silently for more than 40 minutes, more observer than leader, and then offered only a vague sense of where he stood, said people in the meeting.

. . [T]here was no evidence that he was playing a major role in the frantic efforts on Capitol Hill to put a deal back together again. . . . [T]he day’s events succeeded most of all in raising questions about precisely why Mr. McCain had called for postponing the first debate and returned to Washington to focus on the bailout plan, and what his own views were about what should be done. . . .

In a nutshell, McCain was simply clueless and useless at the meeting. I look forward to tonight's debate.