Privacy advocates are concerned that the Obama administration might be prodding Europe to weaken its privacy policies in the name of preventing terrorism.
The privacy groups are worried about negotiations between the Department of Homeland Security (DHS) and the European Commission on an "umbrella agreement" that would combine existing pacts on sharing financial information and airline passenger records.
The talks, which got under way in Hungary last month, are meant to a deal with what data governments can share during anti-terror and crime-fighting operations.
Eleven privacy and civil liberties groups signed the letter, including the American Civil Liberties Union (ACLU), the Center for Digital Democracy and the Consumer Federation of America.
European privacy laws are the "gold standard" for data protection. Other countries will use the pact between the U.S. and Europe as a model for their own privacy rules, so it's essential to set a tough example, they added.
The groups called on Sens. John Kerry (D-Mass.) and Dick Lugar (R-Ind.) to hold hearings on the topic.
The negotiations with the European Union are being conducted by U.S. agencies including DHS and the Department of Justice.
In a joint statement announcing the talks in March, the U.S. and the E.U. said the negotiations will build on the current data-sharing agreements.
"The United States and the European Union are committed to ensuring a high level of protection of personal information, while fighting crime and terrorism," the statement said. "The United States and the European Union are strongly determined to reach, without delay, an agreement that will advance our mutual goals."
This post has been updated.