He started out as a rich guy buying the Governorship, it was easier for him to just bypass the party primaries and run as an indie. He wouldn’t have won the rat primary in 1994 over Joe Brennan. He possibly could have won the GOP primary (over Collins and others).
Given that they almost all endorsed him anyway be probably would have easily won the rat primary for Senate in 2012. Since most of us thought we’d be taking the Senate he probably wanted to hedge his bet. I bet he’s an ego-maniac.
I doubt very much he’d ever joined the GOP, as you say it would just be organizing with us. But unless he promised he wasn’t going back to the rats if they gained a few seats next time, I’d tell him to go screw.
I was in Maine the night he won the Governorship 20 years ago (yikes, was it THAT long ago ?). Maine was one of only two states with open Governorships that flipped away from the GOP (Alaska being the other), although it wasn’t much of a surprise, since the GOP nominee (Susan Collins) was running a distant 3rd (I believe she received the lowest percent for a Republican nominee for Governor in that election cycle, a paltry 23%, only to be outdone by the hapless ex-Congressman Jim Longley, Jr. in 1998, who didn’t even get 19%, although the Dem candidate got a jaw-dropping 12% (!).
Still, it was not considered a “hold” for the GOP (outgoing liberal Republican Jock McKernan was no prize... it’s curious he’s never sought elective office again).
Had King run in one of the party primaries, as a liberal, it would’ve made more sense for him to run as a Democrat, although perhaps he could’ve gotten the GOP nod. Ultimately, I believe he took more votes from the GOP side than from the Dem side (although he still edged out the leftist Joe Brennan by just 1 1/2%).
He probably could get away with caucusing with the GOP without formally switching parties, though the likelihood is that there will be pressure by the ME GOP to run a desultory nominee in 2018, since if they don’t, if the Dems run a strong nominee, they’ll take the seat back with a plurality of the vote.