Both the Democrats currently in the race are arguably stronger than Capps would have been, Salud Carbajal especially. This just makes it less likely for two Republicans to slip into the general.
Also Republicans thinking that they'll pick up this seat in a presidential year is cute. Run a non-joke GOP candidate in 2018.
I mean, it's not very likely. But Republicans do need to spread their eggs into as many baskets as possible. This one has been close for a long time (even in Presidential years, where Capps ran behind the President), so it doesn't make sense to just not even try for it.
