The former vice-president Al Gore launched a withering attack on the White House yesterday for authorising wiretaps without court oversight, and accused President George Bush of repeatedly breaking the law.
The strongly worded speech makes Mr Gore the most prominent political figure in America to weigh in on the wiretapping scandal. Mr Gore, who lost the 2000 election to Mr Bush following the intervention of the supreme court, also went further than other Democratic critics in accusing the president of wrongdoing.
The revelation last month in the New York Times that Mr Bush signed secret orders in 2002 authorising the National Security Agency to monitor the email and telephone calls of thousands of Americans has outraged members of Congress and the judiciary.
Mr Gore said yesterday that the decision to bypass the courts was part of a pattern of behaviour from the Bush administration of "indifference" to the constitution. "We still have much to learn about the NSA's domestic surveillance. What we do know about this pervasive wiretapping virtually compels the conclusion that the president of the United States has been breaking the law repeatedly and insistently," Mr Gore said in a speech delivered to mark Martin Luther King day.
"A president who breaks the law is a threat to the very structure of our government," he said.
Since the 2000 presidential elections, Mr Gore has occasionally used his peculiar position in American politics - he was defeated by Mr Bush despite winning more votes - to advance an agenda that is more liberal than the Democratic party leadership. He has been a far more outspoken critic of the Iraq war than most senior Democrats.
In yesterday's speech, Mr Gore also called for an independent counsel to investigate the secret wiretap programme. He ranked the operation with other controversial decisions by the administration in the war on terror, including its holding of "enemy combatants" indefinitely without trial, and its justification of harsh interrogation techniques.
"The disrespect embodied in these apparent mass violations of the law is part of a larger pattern of seeming indifference to the constitution that is deeply troubling to millions of Americans in both political parties," he said.
Mr Bush insists that he acted within the law and that Congress implicitly authorised the eavesdropping when it allowed the use of force in response to the 9/11 terror attacks. However, yesterday's broadside from Mr Gore increases the pressure on the White House to offer a fuller explanation of its decisions.
The Senate judiciary committee plans to hold hearings next month into the legality of the NSA eavesdropping, and the Republican chairman, Arlen Specter, has indicated that he is sceptical of the Bush administration's assertions that it acted within the law.