Those with experience in national campaigns agree these events have the potential to sway the outcome of a presidential contest that remains highly competitive.
McCain seemed to have scored a tactical victory in the lead-up to his first meeting with Obama, once again shaking up the race and perhaps providing himself with new talking points tonight.
A central theme of McCain's campaign is that Obama has consistently put politics ahead of solving the country's problems.
McCain announced Wednesday that he was setting politics aside and requested a delay in the first debate unless Washington reached agreement on a financial bailout plan. Bush followed by inviting Obama and McCain to a bailout summit.
Those moves forced Obama to abandon debate preparations in Florida and return to Washington.
At the same time, McCain's decision successfully obliterated an unfavorable news story - an ABC News/Washington Post poll that showed Obama with the first clear lead of the campaign. The poll credited Obama's advantage in dealing with the economy for the apparent shift, though the results were far out of line from other national surveys and even Democrats said the numbers were skewed by an unreliable voter sample.
According to the most recent Gallup Daily Tracking poll, Obama and McCain are tied at 46 percent each. Wednesday night's interviewing results did not show that McCain had done dramatically better against Obama after announcing that he was suspending his campaign, Gallup found.
McCain and Obama are scheduled to meet twice more, on Oct. 7 and 15. Vice presidential nominees Sarah Palin and Joe Biden meet next Thursday.
Several strategists made the case that the initial debate would be the most influential.
"The first debate is the most critical one, because Barack Obama is still an unknown quantity. He is largely a cipher to the voters who are still up for grabs," said a Democratic strategist, who requested anonymity because he is not authorized to speak for Obama's campaign.
"Watching him side-by-side with McCain will answer a lot of questions for those folks," he added. "Barack Obama still needs to prove to those voters who are up for grabs that he can do the job as president."
Paul Maslin, a Democratic pollster, called tonight crucial to both men. He said Obama needed to cross a threshold of "reassurance and acceptability" to voters.
