California still hasn’t counted all its ballots from November 6, 2012. So far, there are 426,876 more votes cast for President than for U.S. Senate. For President, voters had a choice of six presidential candidates listed on the ballot, plus write-in space. But for U.S. Senate, they could only vote for a Democrat and a Republican, with no write-in space.
Of course, it is normal for some voters to vote for President and then no other offices in California and all other states. Nevertheless, it is reasonable to believe that many more votes would have been cast for U.S. Senate in California this year if voters had had more choices.