Your question, "If McCain had a 20-year relationship with a disreputable character, would the Obama campaign make hay of the situation? Of course it would" say you,
Well Obama campaign did not, but MSNBC Countdown with Keith Olbermann would and did the evening of Oct 8, 2008.
McCain has many disreputable characters in his closet, including association with terrorists. His association in the 80's with white supremists and his association which is ongoing from the 80's with a woman who supports the bombing of clinics that provide legal abortion. At the rally he was warned not to attend, but did, the woman who bombed a clinic and shot a navy vet doctor in both arms, was called a great woman. (She was convicted as a "terrorist"). The woman who praised her actions and supports bombing clinics (terrorism) has been in McCains camp since the '80's. Interesting videos, you should contact Keith Olbermann show and see the McCain of the 80's if negative truth is what you seek.

Sir; do you know the truth? The ugly truth, or any truth?... I have found life is so much easier when people are willing to tell you the truth; so i have been looking for you all of my life.... I don't expect the truth to be pretty, or proud. I don't expect it to shout out loud. I expect to find it under a rock or in the grave or locked in a vault. For truth is money and money is truth, and if you pay me enough we can  both cut loose with limitless truth, ugly and free with you for a mother and a father for me...  If I thought you were really a person like me, and not an object of sympathy cranking out sabotage for the political feast as if it were not rusty and weak and loud with creaks for simple money to pay your  way and get you through another day dreaming about mo mon eeee....then you  might be the man for me....I never get enough of the truth... I'm not sure I get any..... Thanks...Sweeney