It's an anti-incumbent vote, with the public at large knowing very little about any of the Republican candidates.
Based on that, it is hard to tell how the race will turn out yet. A Republican running for statewide office in California generally cannot win on anti-incumbent sentiment alone.
Campbell has what it takes to win, imo, but it's going to be that much harder to beat a democrat in a federal race, rather than a statewide race. He would have had a much easier time becoming governor. Whitman is a worse candidate than him and even she has a good chance of becoming governor, and depending on how Brown runs, she might even be the favorite.
Campbell sounds too intellectual to win, from what I've heard from debates and speeches. I don't see how he could appeal to Latinos. I have no idea how many lower income whites who are culturally conservative and uneducated but still vote for Democrats still exist in California but I could see Campbell underperforming in areas that contain these voters.