For Musk, who has called Bezos’ company “Sue Origin,” this fight is personal and years in the making. In an early Thursday morning post on his platform X, he gleefully accused Bezos of telling others to sell their SpaceX and Tesla stock because Trump would lose the election. Tesla stock prices have soared since news of Trump’s victory.

The president-elect’s appearance in Texas demonstrates the growing closeness between the two billionaires, and Mr. Musk’s increasing influence in Mr. Trump’s orbit.