Trump is (some) Republicans' "Picture of Dorian Grey." After decades of blissful ignorance, they can suddenly, for the first time, begin to fathom how the rest of the country (and the planet) sees and has seen them going back certainly to W, or even to Nixon, among those who pay closer attention.
They are shocked! (shocked!) and confused. But they clearly haven't been paying attention (or haven't dared to pay attention). And if they reflect, sincerely, it's perfectly easy to understand how their party has ended up here. It is the natural next step of a long-established progression. It's hilarious some of them still insist that they are the "Party of Lincoln" when they are demonstrably Lincoln's diametric opposite. They have embraced this role wholeheartedly, and without a peep of protest from those now aghast at Trump, since 1968, when the South decided it could never forgive the Democrats for ending apartheid and lynching, and Nixon was there waiting to soothe them with the dulcet tones of the dog-whistle.
For the rest of the world, Trump is not a Republican aberration; quite the opposite. He is the Party's slightly more-concentrated and less-refined essence. And the Republicans haven't been able to win elections since Eisenhower without resorting to what he represents.
Republicans who are now horrified with him simply have not been honest with themselves about their Party.