He owes McCain and apology because this is how he plans to run his campaign:

After being forced to respond to three separate incidents in recent weeks of conservatives alluding to Barack Obama’s middle name, John McCain’s campaign manager today sent a memo to top supporters urging them to stick to the campaign’s preferred message — and to avoid taking gratuitous shots at their Democratic rivals.
“Overheated rhetoric and personal attacks on our opponents distract from the big differences between John McCain’s vision for the future of our nation and the Democrats,” wrote Rick Davis in a document the campaign emailed to Republican officials and staffers this afternoon. “This campaign is about John McCain: his vision, leadership, experience, courage, service to his country and ability to lead as commander in chief from day one.”
McCain, Davis added, “has held himself to the highest standards and he will continue to run a respectful campaign based on the issues.”
“We expect that all supporters, surrogates and staff will hold themselves to similarly high standards when they are representing the campaign.”

Maybe Obama can stop the personal attacks against Republicans and join McCain by setting a new tone 🙂