Some Democrats have said they never approved a domestic wiretapping programme, undermining suggestions by the US president and his senior advisers that the plan was fully vetted in a series of congressional briefings.
Jay Rockefeller, the Senate intelligence committee's top Democrat, said in a handwritten letter to Dick Cheney, the vice-president, in July 2003: "I feel unable to fully evaluate, much less endorse, these activities.
"As you know, I am neither a technician nor an attorney."
Rockefeller is among a small group of congressional leaders who have received briefings on the administration's four-year-old programme to eavesdropping, without warrants, on international calls and emails of Americans and others inside the United States with suspected ties to al-Qaida.
The government would still seek court approval to snoop on purely domestic communications, such as calls between New York and Los Angeles.
Some legal experts described the programme as groundbreaking.
And until the highly classified programme was disclosed last week, those in Congress with concerns about having the National Security Agency spy on Americans raised them only privately.
Bush, accused of acting above the law, issued on Monday a forceful defence of the programme he first authorised shortly after the attacks of 11 September 2001.
Despite the defence, there was a growing storm of criticism in Congress and calls for investigations from Democrats and Republicans alike.
"I feel unable to fully evaluate, much less endorse, these activities"
Senator Jay Rockefeller,
the Senate intelligence committee's top Democrat
Until the past several days, the White House had informed only Congress' top political and intelligence committee leadership about the programme that Bush has reauthorised more than three dozen times.
The spying uproar was the latest controversy about Bush's handling of the "war on terror".
It follows allegations of secret prisons in Eastern Europe and of torture and other mistreatment of detainees, and an American toll in Iraq that has exceeded 2150.
Bush's support among Americans has picked up sharply in the wake of Iraq's election and a series of aggressive defences of his security policy, a new Washington Post-ABC News poll showed on Monday.
After a long and deep slump, Bush's popularity rebounded steeply in the past month, with 47% of people voicing overall approval for the president, while 52% disapproved, The Washington Post reported on its website.
That compared to just 39% approval in the Post/ABC poll at the beginning of November.
Bush has seen a rise in popularity
since the elections in Iraq
Likewise, there was a surge in support for Bush's handling of Iraq, which had been key to the decline in his popularity over the past year.
In the newest poll, 46% voiced approval for his Iraq management, 10 percentage points up from November.
And in fighting terrorism, a majority of 56% gave Bush support, compared to 48% in November.
The newspaper noted that the surge in favourable views of Bush's administration came largely from conservatives and Republicans, his traditional support base.