Sen. Patrick Leahy says he'll consider removing mandated DNS-redirects from the Protect IP Act. Photo: AP

Sen. Patrick Leahy (D-Vermont) said Thursday he is having second thoughts on a major provision of a bill he is sponsoring that would force changes to internet infrastructure to fight online copyright and trademark infringement.

The legislation under the Protect IP Act mandates that ISPs alter records in the net’s system for looking up website names, known as DNS, so that Americans couldn’t navigate to sites the attorney general deems are infringing. Critics, including some of the net’s leadi...