WASHINGTON - Some of the toughest questioning of John Brennan, President Barack Obama's nominee to head the CIA, may come from the president's fellow Democrats, not hostile Republicans, at his Senate confirmation hearing on Thursday.

Brennan, 57, is expected to be examined closely about US spy activities from waterboarding to the use of drones when he appears before the Senate Intelligence Committee at the hearing scheduled for 2:30 pm EST/1930 GMT on Thursday.

Even so, there has been no groundswell of objections to Brennan's nomination, and he is expected to win confirmation from both the panel and, later by the full US Senate.

Some of the more vocal concerns about Brennan have come from liberal Democrats, not the conservative Republicans whose reservations  about Obama's nominee to head the Pentagon, Chuck Hagel, prompted the delay of a vote on the former Nebraska Republican senator's confirmation as secretary of defense.

On the eve of the hearing, Obama directed the Justice Department to provide congressional intelligence committees access to a classified opinion laying out the legal basis for armed drone strikes on US citizens alleged to be involved in terrorist plots, an administration official said.    

The White House had previously resisted taking this action, angering lawmakers including Senator Dianne Feinstein who chairs the Senate Intelligence Committee.

Feinstein, a California Democrat, said she was pleased about Obama's decision on Tuesday. "It is critical for the committee's oversight function to fully understand the legal basis for all intelligence and counter-terrorism operations," she said.    

Brennan is Obama's counterrorism adviser and was a CIA executive under former President George W. Bush.

He first surfaced as an Obama CIA nominee in 2008. He withdrew after human rights activists protested against his public statements about the agency's use of "enhanced interrogation" techniques, including the simulated drowning practice known as waterboarding, which a wide range of authorities regard as torture.

Oregon Senator Ron Wyden, a Democratic member of the intelligence committee, has pledged to press Brennan on the Obama administration's use of armed drones to attack and kill militants. The Capitol Hill newspaper Roll Call reported that Wyden had threatened to delay Brennan's nomination to help press his demands for information.

Republicans, however, praised the drone program.

"The drone program to me is a logical use of how you deal with an enemy combatant," said Senator Lindsey Graham after a news conference on the defense budget.

Graham, one of the Republican senators most vocally opposed to Hagel's appointment, said he is "totally supportive" of the administration's rationale for using drones.

Brennan has been interviewed in connection with US prosecutors' probes into unauthorized leaks of government secrets to the news media, according to his written submission to the intelligence panel.

He said he had been advised that he is only a witness in the inquiries.