A Justice Department memo that surfaced this week providing legal justification for the Obama administration's use of drone strikes against American citizens suspected of plotting terrorist attacks stands in stark contrast with the platform Barack Obama ran on in 2008 — and the civil liberties he championed as a young, liberal state senator in Illinois.

The memo, which was first reported by NBC News, argues that the United States government can legally use drone strikes to kill American citizens without due process if they are determined to be high-ranking al-Qaeda officials who show "an imminent threat of violent attack against the United States."

"I can just say that this president takes his responsibilities very seriously, and first and foremost, that's his responsibility, to protect the United States and American citizens," White House Press Secretary Jay Carney said Tuesday, defending the drone policy. "These strikes are legal, they are ethical, and they are wise. The U.S. government takes great care in deciding to pursue an al-Qaeda terrorist, to ensure precision, and to avoid loss of innocent life."

But for a politician who made his name in the state Senate by fiercely advocating reforms to the justice system — including mandatory recordings of all police interrogations and confessions in capital murder cases — and later decried the Patriot Act as an assault to civil liberties, Obama's defense of such drone attacks represents a remarkable departure from principles he championed not long ago.

As early as 2002, Obama was publicly carving out a decidedly progressive stance on these issues, using a Chicago television appearance to defend the civil liberties of American terror suspects who were being detained indefinitely without charges.

"There always has been a distinction between citizens and non-citizens," he said. "It means something to be a citizen. And that's important."

"I'm always more concerned about encroachment on civil rights or civil liberties that apply selectively to people. When they apply to everybody, there tends to be a majoritarian check," Obama added.

By comparison, in a Tuesday briefing, Carney said, "U.S. citizenship alone does not make a leader of an enemy force immune from being targeted."

Similarly, when George W. Bush signed the controversial Military Commissions Act of 2006, he did so at a desk with a sign on the front displaying the words "Protecting America." Obama was a vocal opponent of the law, which was criticized for its broad definition of an enemy combatant, its justification of torture, and its apparent encroachment on habeas corpus rights.

"I'm still disappointed, and I'm still ashamed," Obama said in a Senate floor speech in September 2006, criticizing the bill. "Because what we're doing here today, a debate over the fundamental human rights of the accused, should be bigger than politics. This is serious."

"But we also know that some have been detained who have no connection to terror whatsoever," Obama said. He added, "As one U.S. commander of Guantanamo told the Wall Street Journal, 'Sometimes, we just didn't get the right folks.' And we all know about the recent case of the Canadian man who was suspected of terrorist connections, detained in New York, sent to Syria, and tortured, only to find out later that it was all a case of mistaken identity and poor information."

"In the future, people like this may never have a chance to prove their innocence," then-Senator Obama said.

Less than seven years later, Obama's own press secretary would use those same words, "protecting America," to defend a drone policy that faces many of the same criticisms as Bush's law did.

When Obama ran for president in 2008, he built much of his messaging around rolling back what he considered Bush's assault on civil liberties. He pledged to shut down the U.S. detention center at Guantanamo Bay and slammed Bush by declaring, "I was a constitutional law professor, which means, unlike the current president, I actually respect the Constitution."

He continued the same tack throughout Democratic primaries and the general election.

In a 2007 questionnaire given to the presidential candidates by the Boston Globe, Obama responded to a question about when it's OK to bomb a sovereign nation by saying, "The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation."

"I also reject the view, suggested in memoranda by the Department of Justice, that the President may do whatever he deems necessary to protect national security," Obama said in that same Globe questionnaire.

But in 2011, White House Homeland Security Advisor and the Obama administration's nominee for director of the Central Intelligence Agency defined "imminent threat" in broad terms that would likely contradict the view of then-Senator Obama.

"Over time, an increasing number of our international counterterrorism partners have begun to recognize that the traditional conception of what constitutes an 'imminent' attack should be broadened in light of the modern-day capabilities, techniques, and technological innovations of terrorist organizations," Brennan said in a speech at Harvard Law School.

And when Carney was asked about the legality of the drone program this week, he cited the president's responsibility "to protect the United States and American citizens."