Start with this: John Mitchel took about 20% of the vote from George Voinovich in 2004's Presidential primary, spent $300 and used no new-media techniques that I'm aware of.
DeWine's 2000 challenger took about the same % in the 2000 primary with virtually no money or New Media.
DeWine only won 52% of the vote in the contested 1994 primary when he was the Lt. Gov. of the very popular (at the time) George Voinovich. Mike DeWine has NEVER been popular with the base, but there has never been a viable alternative; NOW THERE IS. Your point about Voinovich is I think weakened by the relative weakness of the candidate he face compared to Sherrod Brown.
Based on past turnouts, the winner next Tuesday will get about 500,000 - 600,000 votes. If Pierce gets 15% of the roughly 3 million GOP registered voters to vote for him Tuesday, that's almost enough, and he has a lot more visibility at the grass roots than you think he does. 20% and he's in.
The point is, it's doable.
Hmm. I'm an Ohioan, and think Tator has a point. If I weren't reading the blogs, I'd barely have heard of Pierce. There's little advertising going on here (southeast Ohio) for Pierce.
But either DeWine or Pierce can certainly beat Brown.