Seeing that Open Secrets seems to be the gold plated source, we need to get this fact out - Obama took almost no PAC money from any company.
http://www.opensecrets.org/pres08/summary.php?cycle=2008&cid=N00009638 PACs are the ONLY way a company can give money. His contributions were almost entirely from individuals.
Although I respect the reason it was originally set up and I understand why the summaries they do are useful, Opensecrets and other sources really do a disservice when they conflate individual contributions with PAC contributions.
Obama raised an unprecedented amount of money - all in contributions less than $4,600 per person (assumes $2,300 in both primary and general election) All of these people work for companies. It is very likely that Obama will have raised tens of thousands of dollars from employees of every company that enters the news. (look at Goldman Sacks - where the right is making the same argument - surprised that a NYC based company had a lot of Democrats working for it? )
Where giving the composite individual and PAC numbers likely overstates the relationship to the company, this likely understates it. I'm not naive enough to think that companies really played this small a role. I do know that there are bundlers and bundler can collect individual contributions and be seen as the fund raiser who brought in that money. If someone high in a company used their position in the company to raise money for a candidate, it is kind of like a PAC, without the legal hassles of being one. It results in a large sum of money raised for which the bundler gets "credit". The worst case would be where the company really does give the money. Remember in I think 2005, one law firm got in trouble by having everyone or almost everyone contribute the max in 2003 or 2004 to Edwards - then reimbursing them. Here, the amounts from BP are not high enough to suggest that there was anything wrong.
Yet, these were not controversial contributions when made. Many were likely made with no particular outreach. Now, why was Obama higher than McCain by a considerable amount? McCain took public financing in the general election - and Obama did not. Do you want to question that decision now? I always thought that public financing was what could end this. However in a world where the Republican candidate starts out with a free echo chamber made up of talk radio and FOX News, even with more money getting our message out is harder. Public financing implicitly assumes a level playing field.
To see the difference made by not taking public financing, look back to what happened on campaign spending in 2008 and compare it to 2004. In 2004, the Kerry campaign had to very cautiously spend money - going nearly off the air in August. This hurt, but the alternative would have been to have considerably less money in the fall than Bush. They also had to pull out of marginal states to concentrate resources in Ohio, PA, and FL - and we know how that turned out. Obama on the other hand had essentially no limits - even spending some money on ads in Arizona. Remember the wonderful 30 minute "ad" bought at the end, where the issues and close ups choosing who Obama was were laid out completely under our control - I think on several channels?
Now, 2008 was not 2004. In 2004, in the week before the election, Gallup found that 59% of the country answered that the country was doing either very well or fairly well. In 2008, only about 20% were answering that way. But, having all that money helped get Obama's message out and helped enough people see him in a positive enough light to be comfortable voting for him. I would argue that with the resources Obama had in 2008, Kerry would have won in 2004. Given the underlying dissatisfaction with Bush, I think Obama would still have won had he taken public financing, but he likely would have lost some of the close purple states. It also would have been a nail biter. (Not to mention, you never know the path not taken. Did the ability to massively respond prevent any slimy attacks? It seems possible to me.)