I'd consider him part of the GOP establishment at least since he came close to taking Ted Kennedy's Senate seat in 1994.
"since he came close to taking Ted Kennedy's Senate seat in 1994."
Edward M. Kennedy, Democrat, 1,266,011, 58.1%.
W. Mitt Romney, Republican, 894,005, 41.0%
Hoo-wee, that was a squeaker, all right. ;)