Texas - March 4
This is kinda important: it's not a primary:
126 of the 193 delegates will be chosen at the district level, but differently than in other states where they’ve been allocated based on Congressional district: they’ll be chosen based on totals in 31 state senate districts by caucuses in more than 18,000 precincts. The remaining 67 pledged delegates will be chosen by statewide primary vote.
So the Texas primary gives only a delegate catch of 67 delegates, less than, say, Minnesota.
On the same date, in the evening, precinct caucuses will be held across the state and to participate one has to have voted in the primary earlier in the day. That is where the big prize of 126 delegates will be chosen.
That scenario favors a campaign that spends the resources and has the volunteers to wage a ground game on 18,000 fronts.