Why were Texans so committed to retaining control of the Tidelands? Texans wanted to retain control of the Tidelands because...

The federal claim was based on allegations that original ownership of the land was held by the united states before california was made a state, that former supreme court cases adjudicating state ownership were erroneous and should be overruled

Basically, Texas has had underwater areas for as long as it has existed. When it was discovered that there were oil deposits there, the federal government wanted to take it away from Texas who was a recognize owner of the land. It was a case where the state and the federation had to fight in the supreme court. Politics changed because Texas started hating on anyone who tried to take it away so they switched their presidential support based on this.