An anonymous Republican senator has delayed a vote on legislation that would require police to obtain a warrant before accessing emails and other online messages.
Leahy had hoped to fast-track the bill to passage with unanimous support, but the opposition means a vote will be delayed until at least September.
A Leahy aide said the senator will continue to work with Republicans to address their concerns. The Senate could pass the legislation without unanimous support, but it would take up valuable floor time to override a filibuster.
It is unclear which Republican or Republicans objected to the bill, S. 607.
Sen. Chuck Grassley (R-Iowa), the ranking Republican on the Judiciary Committee, has expressed concern about how the bill would affect civil regulatory investigations, but Beth Levine, a Grassley spokeswoman, said he was not the one to place a hold on the bill.
Privacy concerns have moved to the front burner on Capitol Hill in the weeks since revelations about the National Security Agency's surveillance programs.
Leahy's bill would not affect the NSA programs, but it would curb the ability of local and federal law enforcement officials to access private online messages.
Under the Electronic Communications Privacy Act (ECPA) of 1986, police only need a subpoena, issued without a judge's approval, to force Internet companies to turn over emails that have been opened, or that are more than 180 days old.
When lawmakers passed ECPA more than 25 years ago, they failed to anticipate that email providers would offer massive online storage. They assumed that if a person hadn't downloaded and deleted an email within six months, it could be considered abandoned and wouldn't require strict privacy protections.
Leahy and privacy advocates argue that ECPA is woefully out of date and that police should need a warrant, based on probable cause and approved by a judge, to read a person's emails.
Privacy advocates argue that the Fourth Amendment already requires a warrant for email searches, but the courts have traditionally ruled that people have limited privacy rights over information they share with third parties. A federal appeals court endorsed a warrant requirement for email searches in 2010, but the Supreme Court has yet to settle the issue.
Leahy's bill has the strong support of Internet companies like Google, Facebook, Yahoo and Microsoft, who argue that the current law erodes trust in online services.
"Users expect, as they should, that the documents they store online have the same Fourth Amendment protections as they do when the government wants to enter the home to seize documents stored in a desk drawer," Richard Salgado, a Google official, said at a House hearing earlier this year.
The Senate Judiciary Committee approved Leahy's bill in April, and a bipartisan group of House lawmakers are working on companion legislation.
But Grassley and some other lawmakers are concerned that the warrant requirement could hinder civil regulatory investigations. Warrants are only available in criminal cases.
Mary Jo White, chairwoman of the Securities and Exchange Commission, sent a letter to Judiciary Committee senators earlier this year, explaining that few of her agency's cases involve criminal charges.
She warned that applying a warrant requirement to the SEC would impede the agency's “ability to protect investors and to assist victims of securities fraud."
But Leahy and privacy advocates argue that regulators can still obtain relevant information by directly subpoenaing the individuals or companies who are under investigation.
They say defendants and their lawyers should be making decisions about when to withhold irrelevant or privileged documents, not a third-party Internet provider who isn't involved in a case.