WASHINGTON—There's nothing neutral in the battle between AT&T Inc. and Google Inc. over the future of the Internet.

Google, the powerhouse of Silicon Valley, and AT&T, champion for the old-line phone industry, are marshaling political allies, lobbyists and—in AT&T's case—labor unions for a fight over proposed "net neutrality" rules that could affect tens of billions of dollars in investments needed to upgrade the U.S. broadband network, which lags in speed and affordability compared with some countries.

On...