The heightened focus on President Obama's targeted killings of American terror suspects overseas has rattled members of his progressive base who have stayed mostly silent during an unprecedented use of secret drone strikes in recent years.

During the presidency of George W. Bush, Democrats, including then-Sen. Obama, hammered the administration for employing enhanced interrogation techniques, which critics labeled torture.

Liberals have hardly championed the president's drone campaign but have done little to force changes in the practice, even as the White House touts the growing number al Qaeda casualties in the covert war.

The issue grates on some Democrats who backed Obama over Hillary Clinton because of her vote in favor of the war in Iraq, only to see the president ignore a campaign promise to close the detainee holding camp in Guantanamo, Cuba, and mount a troop surge in Afghanistan.

With the confirmation hearing Thursday for John Brennan, Obama's nominee for CIA director -- and the architect of the drone program -- Democrats will have a high-profile opportunity to air their concerns over the controversial killings.

"You watch and see -- the left wing of the party will start targeting Obama over this," said Larry Sabato, a political scientist at the University of Virginia. "It's inevitable. The drumbeat will increase as time goes on, especially with each passing drone strike."

Obama late Wednesday decided to share with Congress' intelligence committees the government's legal reasoning for conducting drones strikes against suspected American terrorists abroad, the Associated Press reported. Lawmakers have long demanded to see the full document, accusing the Obama administration of stonewalling oversight efforts.

Earlier in the day, one Democrat even hinted at a possible filibuster of Brennan if given unsatisfactory answers about the drone program.

"I am going to pull out all the stops to get the actual legal analysis, because with out it, in effect, the administration is practicing secret law," said Sen. Ron Wyden, D-Ore., a member of the Senate Select Intelligence Committee. "This position is no different [than] that the Bush administration adhered to in this area, which is largely 'Trust us, we'll make the right judgments.' "

In a Justice Department memo released this week, the administration argued it could order the killing of a suspected American terrorist even with no imminent threat to the homeland.

White House press secretary Jay Carney insisted on Wednesday that the administration had provided an "unprecedented level of information to the public" about the drone operations. Yet, questions remain about who exactly orders the killings, or even how many operations have been conducted.

"There's been more noise from senators expressing increased discomfort [with the drone program]," said Joshua Foust, a fellow at the American Security Project. "For Brennan, there's going to be more opposition from Democrats than Republicans. It's not just drones but the issue of torture."

Facing concerns from liberals, Brennan had to withdraw his name from the running for the top CIA post in 2008 over his connections to waterboarding during the Bush administration.

Since becoming president, Obama has championed and expanded most of the Bush-era terror practices that he decried while running for the White House in 2008.

It's estimated that roughly 2,500 people have died in drone strikes conducted by the Obama administration.

However, most voters have embraced the president's expanded use of drone strikes. A recent Pew survey found 62 percent of Americans approved of the U.S. government's drone campaign against extremist leaders. And some analysts doubted whether Democratic lawmakers would challenged Obama and risk undermining his second-term agenda.

"Democrats, they're going to want the president to succeed on domestic priorities and don't want to do anything to erode his political capital," said Christopher Preble, vice president for defense and foreign policy studies at the Cato Institute. "It's just so partisan right now. An awful lot of [lawmakers] think the president should be able to do whatever he wants."