Topeka U.S. Sen. Sam Brownback, R-Kan., on Friday said the Bush administration needed to answer questions about spying on Americans without court authorization.
And Brownback said he disagreed with the administration's legal rationale, which he said could hamper future presidents during war.
"There are questions that should be examined at this point in time," Brownback said during a news conference.
Bush has confirmed that he approved allowing the National Security Agency to monitor Americans without seeking warrants from a secret federal court that oversees the 1978 Foreign Intelligence Surveillance Act, or FISA.
Bush said the move was necessary to fight the war on terror.
The administration said Bush's decision was legal in part because of a congressional resolution that authorized force to fight terrorism, which was adopted after the 9-11 terrorist attacks.
Brownback said he disagreed with that justification.
"I do not agree with the legal basis on which they are basing their surveillance - that when the Congress gave the authorization to go to war that that gives sufficient legal basis for the surveillance," he said.
He said if the justification holds up, "you're going to have real trouble having future Congresses giving approval to presidents to go to war."
Earlier this week, U.S. Sen. Pat Roberts, R-Kan., chairman of the Senate Intelligence Committee, acknowledged he was aware of Bush's secret operation, and that he agreed with it and thought it was legal.
Brownback said he wasn't opposed to the administration conducting surveillance but that the legal basis had to be straightened out.
He said of Roberts, "My colleague Pat Roberts is doing an outstanding job on intelligence. It's a tough issue."
And he defended Roberts against criticisms from top Democrats on the Intelligence Committee who have said they had problems with Bush's operation.
Brownback said the controversy was becoming politicized.
He said the best way to examine the issues was to have hearings when Congress reconvenes in January. Brownback is a member of the Senate Judiciary Committee, whose chairman, Sen. Arlen Specter, R-Pa., has already announced that he plans such hearings.
In a related development, it was revealed Friday that Supreme Court nominee Samuel Alito advocated a strategy to strengthen the government's power in ordering domestic wiretaps when he worked for the Reagan Justice Department in 1984.
Asked whether that position espoused by Alito should be part of his confirmation hearing, Brownback, who supports Alito's nomination, said it should.
"Anything in his record is subject to the hearing and review and should be," Brownback said.