Today is the deadline for candidates to file for the Texas and California primaries. If there were a Presidential dark horse that wanted to take it at the convention, he or she would enter both of these primaries, perhaps the only ones they did enter, and go ahead and win them. Thus, everyone would be convinced they are the best candidate.
Tomorrow a number of county Republican parties will meet to discuss endorsements. I believe the Democrats centralized things at their party convention. I'll likely go through the candidate lists some time in the next week. Republicans will have state senate candidates in key districts.
The June California Presidential primary might be meaningful. That might mean higher Republican turnout and Democrats not being motivated to go to the polls. In California's Top Two Democrats and Republicans will be competing on the same ballot, which is new due to Prop. 14. If that's the case, it could benefit Republican congressional and legislative candidates.