Senators serve 6 years, elections are scheduled for one third to run for office every 2 years. Every election has senators up for relection, but not all of them. The idea was that this way it would be hard for a popular (but dangerous) movement to take over the gov. The house is elected every 2 years and is much more responisive to the people because it it. (that is not always a good thing!)

Sadly neither of the CA senators can be voted out this election.