I would not discount him. If it weren't for the dang runoff rule, I would have this as Democrat's most likely red state upset. He's well positioned to get crossover votes from anti-establishment Trump indies too. Populist outsiders who've never or only barely held elected office are exactly what has worked in red/blue state upsets recently. Think Scott Brown, Hogan, Baker, JBE, Ernst, Donnelly, Tester, Hagan over Dole, Wolf (blueish swing state, but should never have defeated an incumbent R in 2014!), etc.
At any rate, he's a far better bet for GA Dems than running more retreads and political families.
You're making some good points and I would be very worried about this race if it was an open seat, but no Republican incumbent with crossover appeal in a Deep South state is going to lose this year. Barksdale spent all that money only to barely avoid a runoff in the primary against two no-name candidates. The white vote is way too polarized for Barksdale to win and I suspect Isakson will outperform Trump among minority voters as well (maybe not by much, but by enough to get >50% on election day). My guess is that Isakson wins by 9 or 10 points (54%-44% or so).
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