"They've chosen confrontation rather than compromise or cooperation," Leahy said. "The bottom line on this U.S. attorneys investigation is that we have people manipulating law enforcement. Law enforcement can't be partisan."

At issue is whether the White House exerted undue political influence in the Justice Department's firing of prosecutors.

Leahy's hardening stance is pushing the Democratic-led investigation closer to a constitutional showdown over executive power and Congress' right to oversight.

The White House accused the committee of overreaching.

"After thousands of pages of documents, interviews and testimony by Justice Department officials, it's clear that there's simply no merit for this overreach," presidential spokesman Tony Fratto said.

He said Leahy "is seeking access to candid and confidential deliberations from the president's advisers - an intrusion he would never subject his own staff to. We have gone to great lengths to accommodate the committee in their oversight responsibilities."

Separately, the Senate has subpoenaed the White House and Vice President Dick Cheney's office for documents related to the administration's legal basis for conducting warrant-free eavesdropping on people in the United States.

Leahy and Rep. John Conyers (D-Mich.), who heads the House Judiciary Committee, have demanded a White House explanation by next Monday as to its grounds for claiming executive privilege in refusing to turn over additional documents.

The two lawmakers say that regardless whether the White House meets the deadline, they would begin acting to enforce the subpoenas as appropriate under the law.