“How many Republicans will defect instead of voting for Trump? Do the numbers offset?”
Only if they would actually vote for Hillary.
Which I doubt. They’re just having temper tantrums.
If Trump is picking up 20% of Democrats you won’t see many detections. The rank-and-file GOPe will mostly rally around him, the GOPe elite will mostly glom on in the hopes of gaining some favor from a Trump Administration.
At a 20% crossover pickup, the only threat of the Dem winning the election would be from Webb’s increasingly likely independent spoiler run. But given that Trump’s popularity is based on his popularity/branding, it’s hard to figure out where Webb would draw more from.