Webb would be losing because the base would loathe him.
Where do you think the base will go? Stein? Trump? Ha.
I would vote for Johnson over Webb.
As for which Democrats would be losing to Trump:
Democrats who ran:
-Chafee (mediocre debater, completely uninspiring)
-Webb (too conservative for the post-Obama Democratic Party)
Democrats who didn't run:
-Debbie Wasserman Schultz (easy to paint as corrupt, very unpopular with the base)
-Anthony Weiner (Carlos Danger)
-Bob Menendez (Isn't he currently being charged with some crime?)
-Joe Manchin (see Webb)
