Sen. Amy Klobuchar (D-Minn.), chair of the Senate Judiciary Committee’s antitrust subcommittee, and Sen. Charles E. Grassley of Iowa, the top Republican on the Senate Judiciary Committee, announced that they will introduce legislation early next week making it illegal for Amazon, Apple, Facebook and Google to engage in “self-preferencing,” the tech giants’ practice of giving their own products and services a boost over those of rivals on their platforms.