A senior Republican has accused US Vice-President Dick Cheney of trying to influence an inquiry into the legality of a domestic surveillance programme.
Mr Cheney got a strongly worded letter from Mr Specter
Senate Judiciary Committee chairman Arlen Specter said Mr Cheney had worked against his efforts to mount a hearing.
President Bush ordered the wiretap programme after 9/11, allowing international phone calls and emails by US citizens to be monitored.
Reports surfaced in May that millions of domestic records were also gathered.
In an open letter to Mr Cheney, Mr Specter accused the vice-president of lobbying other Judiciary Committee members to dissuade them from holding a hearing.
"It is neither pleasant nor easy to raise these issues with the administration of my own party, but I do so because of their importance," he wrote.
"I was advised... that you had called Republican members of the Judiciary Committee lobbying them to oppose any Judiciary Committee hearing - even a closed one - with the telephone companies."
Mr Specter, who has threatened to issue subpoenas to force witnesses to testify, said he hoped to avoid " a constitutional conflict between the Congress and the president".
WHAT IS THE NSA?
US government intelligence service founded as a code-breaking agency in 1952
Intercepts communications using satellites and bugs
Said to be largest employer of mathematicians in the US
Budget and staff size classified
An agency as a whole so secretive its initials are said to stand for "No Such Agency"
Last month, allegations surfaced that the National Security Agency had asked all the major phone companies for access to their records of all calls.
The senator wants to question telephone company executives to find out to what extent they contributed to the surveillance programme.
The domestic spying programme allows the NSA to monitor international telephone calls and emails of US citizens without obtaining a warrant if it is in pursuit of al-Qaeda suspects.
Critics argue that the programme violates the 1978 Foreign Intelligence Surveillance Act (FISA), which requires approval from a secret federal court for domestic monitoring involving US citizens.
Mr Specter wants the NSA programme to be subject to a review by the court.
Last month, President Bush defended the surveillance programme, saying that all intelligence activities he authorised were lawful and targeted al-Qaeda.
A spokeswoman for Mr Cheney said the vice-president had not yet studied the letter but that the White House was prepared to consider congressional measures to bring the programme under federal law, Reuters reported.