In 2009, the off year California voter registration report had Democrats at 44.5% of registered voters and Republicans at 31.1%. Every subsequent report for the next three years through the September 7, 2012 report had Democrats and Republicans losing voters at similar clips. On September 7, 2012, the numbers were 43.3% Democrats/30.1% Republicans. In 3.5 years the Democrats had dropped 1.2% and the Republicans dropped 1.0%. After the September 7 report, California implemented online voter registration. While both parties added voters, Democrats added them at a much faster pace and the October 22 report had Democrats at 43.7% to Republicans 29.4%. There were a lot of people who wanted to be Democrats who Democrats had been unable to register. Online registration provided a big boost to Democrats in November 2012, as these new voters voted at a higher rate than those registered before. The extra Democratic voters may have made a difference in several races, helping Democrats win more than people thought.
Will online registration be enough to help Democrats keep making bigger gains? No one knows for sure, although the next registration report, which should come out any day, should be enlightening. Democrats were hoping that same day voter registration would provide a similar boost. Currently a citizen must register to vote several weeks before an election in order to be eligible to vote. Same day registration would allow a voter to show up at the polls, show some form of proof of residency and then vote provisionally. The state would subsequently check to see if they were actually eligible to vote at the address they gave. Democratic thinking is that their voters would be more likely to do this than Republican voters. Other states have same day registration, but I'm not aware of any evidence showing who it helps.
There were some legal requirements the state had to achieve in order for same day voter registration to be implemented. The state hasn't met them, so there won't be same day voter registration in 2014. I don't know if Republicans should be relieved but they likely are.