My first thought was "Harder? These people seem to be completely out of touch with what's been going on for the last month. What's harder than going 100% negative? What's harder than accusing him of being a terrorist and a traitor, and giving your followers your tacit approval when they scream out things like 'Kill Him!' during rallies?"
The only way he could get nastier with Obama would be to start taking these accusations up in his media interviews and the last debate. And I'm sure Obama would like nothing more than for McCain to do that. It would be the last nail in the coffin of the GOP campaign.
John McCain cannot win a war of guilt-by-association in any arena where people who read the newspaper and know anything about his history are to be found. He brings up Bill Ayers, an old, irrelevant, forgotten and failed 60's revolutionary who Obama only knows in passing, and Obama comes back with Charles Keating, who McCain actively abetted in the former's attempts to subvert thrift regulation. McCain brings up Jeremiah Wright, Obama counters with John Hagee. McCain brings up ACORN, and Obama can again counter with the Keating Five.
I'm not particularly surprised that people who would attend a McCain/Palin rally would think that this strategy would work. Between the media isolationism the right has been indulging in in the last 2 decades, and their more recent war on intellectualism, the GOP has been working very hard to produce an electoral base of mindless robots. They got what they wished for, bless their little hearts. And I do mean little.
Taking this tack would be suicide for McCain. And while McCain has had a notable talent for blunders recently, I don't think even he is clueless enough to commit this one. But he could yet prove me wrong.