If, in 2017, you thought, 'Corbyn's my guy!', then in 2019 thought 'Boris Johnson's my guy!', you probably misunderstood something or weren't paying attention at some point.
Not dumb, they just misunderstood because they were not paying attention, and so ended up getting stockholm syndrome. They need to be cured / re-educated. Or maybe just replaced.
If, in 2017, you thought, 'Corbyn's my guy!', then in 2019 thought 'Boris Johnson's my guy!', you probably misunderstood something or weren't paying attention at some point.