(CNN) You're going to hear a lot about something called the "top-two" primary in the lead up to California primary. Tuesday's contest in the Golden State has been described as a potential "disaster" for Democrats in their march to pick up a net gain of 23 House seats and the majority in November.

And while that could eventually be the case, the truth is we have so little data as to be almost totally in the dark when looking at these races, and a look at the little data that is available suggests that such fears are probably overblown.

To understand why Democrats worry about the top two, you first have to realize what's going on.

Simply put, all the Democratic candidates in a district and all the Republican candidates in a district run against each other in a primary, regardless of party affiliation. The two top vote getters advance to the general election in November, even if they are members of the same party. If there are more members of one party running than the other, it strengthens the chance that even if a particular party has fewer supporters its candidate can advance to the general election because so-said party is splitting its votes among fewer candidates.

It's this vote splitting among many different candidates of the same party that is the big fear for Democrats in three Republican held districts : California 39 (where Ed Royce is retiring), California 48 (where Dana Rohrabacher is running for re-election), and California 49 (where Darrell Issa is retiring). These are all districts Hillary Clinton won, so they're seen as big Democratic pickup opportunities.