For another Democrat to defeat a Democratic incumbent, said Democrat would need to pick up a significant share of the right-leaning vote. That aside, it's not so much that conservatives support De Leon, they are just selecting the option that is not Feinstein.
It should also be noted that they might well switch back to Feinstein the minute De Leon is actually in striking range.
After all, those conservatives just lurved them some Obama...then he won Iowa and got ahead of Hillary. No shock that suddenly, Fox was tooting the horn for her in ways little different from all the goodwill McCain had on the left before he picked Palin.
Ultimately, that's probably what's going on here.
