McCain Endorsement Valuable For Romney?!?
The 'short answer' would be not-so-much! The reverse of this happened in the 2008 GOP campaign after John McCain handed Mitt Romney his ass--verbally--during those presidential debates.
Each has accused the other of dirty politics--as is the norm--as well as flip-flopping on the issues.
As applied in today's politics, this endorsement may actually be a detriment. How much benefit would a liberal RINO's endorsement of a flip-flopping RINO actually be?!?