WASHINGTON—Federal antitrust enforcers and lawmakers are poised to scrutinize the nation’s largest technology companies for potential anticompetitive practices, bringing a new regulatory focus to the vast markets for digital services and a new level of concern for investors.

After years when the government took a broadly laissez-faire attitude toward the regulation of Silicon Valley, antitrust officials at the Justice Department and Federal Trade Commission are choosing lanes with a view to studying the practices of at least...