[H]istorical precedent is against such a move: even the desperate Herbert Hoover didn't change horses in 1932, despite knowing he faced certain defeat. Others have changed vice-presidents in the last 100 years, but never for desirable reasons: it happened in 1944 because Henry Wallace, as vice-presidential candidate, succeeded in alienating everyone; and in 1976, because Nelson Rockefeller actually refused to run. The real reason veep switches don't happen often is because vice-presidents don't make much of a difference.
Massie is on the same page:
It projects weakness which, generally speaking, is the kind of thing a candidate seeks to avoid doing. It's the kind of thing that changes the lens through which the media filters its reporting.