AT&T Inc. Chief Executive Randall Stephenson made an $85 billion wager last month that would turn the giant telephone company into one of the world’s biggest media companies by swallowing Time Warner Inc.

The same day, Donald Trump told supporters in Gettysburg, Pa., he would block the deal if elected president. “It’s too much concentration of power in the hands of too few,” he said, calling the merger “an example of the power structure I am fighting.”

Few...