Ellipsis....the Republican Party had every opportunity to drop Trump months ago, but decided to carry on arguing amongst themselves, in a vain hope he would go away.
So, to drop him with just 4 weeks to go, would show everybody that they made a mistake in the beginning....plainly speaking....that they cocked things up, big time.
Few of the big, main players in the Party now think that Trump will win, but it isn't just the President that Americans will be voting for on November 8th.....its a heap of other posts as well.
https://en.wikipedia.org/wiki/Election_Day_(United_States)
If they admitted to the electorate, that they have been so stupid as to allow such an unsuitable candidate to go forward as the Republican candidate, by withdrawing him, all the other Republicans in line to be voted for on the same day, would then be in danger of losing as well.
In other words, its damage limitation time for the Republicans. When Trump loses, they will, at least, have the structures in place to re-gather their forces, to fight again in 4 years time.
But I have to point out that the Republican Party seems to have lost its way over the last few years. They hoped to come back after their defeat in 2008, and 2012, but they seem to have learned nothing from past defeats.
In 2012, its was the "Magic Underpants" effect, and now its Trump-Gate.