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.
Jim Webb is going to end up playing the part of Zell Miller at the 2016 GOP convention in return for a high-level Defense Dept. post.
At the same convention (if the GOP has any sense), Chris Christie will give a prosecutor's speech summarizing all of the lies Hillary Clinton has told in her lifetime. If the Republican candidate wins, he deserves to be rewarded with the AG post.