If they are going to grow one, they'll do it around July. The only way to salvage the midterm election is to keep sucking up to Trump until the primaries are over, then do a complete 180 and attack him with everything they've got. Impeachment hearings, everything. They have to appear more anti-Trump than the Democrats if they want to keep the existing Dem wave from becoming a tsunami.
But they can't do it until then, because their dwindling base is still conditioned to support whoever the most rabidly insane candidate appears to be. If they jump the gun they'll get primaried by someone who isn't just acting crazy.