Turns out that McCain also delved into murky area of loaning money to his campaign, albeit in 1982. He loaned $167,000 from his wife's fortune to break into politics and win a contested Republican primary in an open seat. That's $376,000 in today's dollars, and has the same ethical problem of subsequent campaign donations repaying the loan and going directly into his pocket. Arizona Democrats looking to dig up some dirt on McCain might research who donated money to his campaign after he won the primary - the district was heavily Republican, so any donations from that point on until the debt was paid, had the sole effect of going into McCain's bank account.
McCain had no other tough Congressional or Senate contests, so I doubt he loaned money again. I was able to check FEC records for his 2000 and 2008 presidential campaigns, and no personal loans are listed.
I suppose it's also worthwhile to ask whether Obama loaned money to any of his early campaigns.