The fact is that the Republican Party is the smaller of the two. If you gut just 20% of it base you have to fight an almost insuperable struggle to pry loose Dems and new voters. It looks as though at least half the establishment is in revolt against Trump, which would make that 20% defection rate seem conservative.
“It looks as though at least half the establishment”
The establishment numbers don’t translate to very many “man-on-the-street” voters.