WASHINGTON — Edward J. Snowden said he had leaked secret documents about National Security Agency surveillance to spark a public debate about civil liberties. President Obama, while deploring the leak, endorsed the same goal of a vigorous public discussion of the “trade-offs” between national security and personal privacy. “I think it’s healthy for our democracy, “ he said on Friday of the prospect of re-examining surveillance policy.

But the legal and political obstacles to such a debate, whether in Congress or more broadly, are formidable. They only begin with the facts that the programs at issue are highly classified and that Mr. Snowden is now a hunted man, potentially facing a prison sentence for disclosing the very secrets that started the discussion that Mr. Obama welcomed.

On Monday, the White House spokesman, Jay Carney, was pressed about just how the surveillance dialogue the president invited might take place.

Asked whether Mr. Obama would himself lead the debate or push for new legislation, Mr. Carney demurred. “I don’t have anything to preview,” he said, adding that the president’s major national security speech May 23, before the N.S.A. disclosures, showed “his interest in having the debate and the legitimacy of asking probing questions about these matters.”