Don't donate to Feinstein or De León if you don't want money to flow into that race. It's not like either of them are anathema to the average California Democrat.
The advantage, if they both make the top two, is that it would depress Republican turnout in House elections, although I question how ethical this strategy is.
If the California Republican party determines that it would hurt them to not have a candidate in November, they will endorse a primary candidate, which will increase the odds that its' Feinstein vs. a Republican, which definitely not be a race worth voting in.
