The California Senate, voting along party lines, advanced a bill that requires all Presidential primary candidates to publicly disclose the previous five years of federal tax returns. They tried this in 2017 but (then) Governor Jerry Brown vetoed the bill, believing it was unconstitutional. Current Governor Gavin Newsom is more of a partisan activist than a Governor and the ruling party believes he might sign the bill into law
California SB27 was co-authored by two Bay-Area Democratic Senators: McGwire from Sonoma County and Wiener from San Francisco. Their reasoning is that the President should disclose his tax return unless he has something to hide. Disclosing personal tax returns is 40-year old tradition in Presidential contests but is not required by law.
The ruling party in California is using this as a form of voter suppression. Next year’s primary election was moved up to March (from June) to overweight California’s interest on the primaries. With close to two dozen Democratic candidates, the March primary could be a boon to Senator Kamala Harris; her candidacy is expected to bring out a high percentage of Democratic voters. On the Republican side, former Massachusetts Governor Bill Weld may be the only Republican on the primary ballot if this bill passes. Most Republican voters support President Trump and would skip the tainted primary election.
All elections are about more than Presidential campaigns. Many county and local contests are decided in the primary election if the winning candidate earns 50.1% of the vote. Additionally, a low GOP turnout and lopsided Democratic turnout could lock more Republican candidates out of run-off elections (like the 2018 76th Assembly District race).
If you hate President Trump, this is a cute way to poke a stick in his eye but, in reality, this is voter suppression, just like the old Democratic party poll taxes in the Jim Crow South.