Fitting Snowden in with the constitutional definition of treason, he would have to:
Give aid and comfort the the enemies of the United States. He only is guilty of that if the American citizenry can be construed as an enemy of the United States, and he gave aid and comfort to them by telling them that the government is spying on them;
Take up arms against the United States. Knowledge is power, but it is not an arm. He took up information and disseminated it;
Making common cause with the enemies of the United States. Once again, this definition only fits if the American citizenry are construed as the enemies of America.
What he did do was disseminate information in order to torpedo a clandestine program by the United States government, which is not the same as the nation called the United States. The government is appointed through a republican process, and given the task of safeguarding its people’s liberties. Governments come and go, and parties are in and out of power at different times. They are not the nation, but its servants (see Menachem Begin, Revolt!, chapter titled Milchemet Achim Leolam Lo [never a civil war{read it in Hebrew, don’t know how it translated in English text}]).
Thus, Snowden was not a traitor. He was a rogue agent of the government, but sometimes one has no other morally acceptable choice than to go rogue. Whether or not Snowden was faced with such a dilemma is something we can each have an opinion on.
Did I miss when Woodward and Bernstein were prosecuted for treason?
The question of Snowden will be a decisive one in the Republican primary. I’m not sure who else besides Rand Paul and Ted Cruz will be positive about Snowden’s leaks. I think both have praised the leaks but neither has said he should not be prosecuted or be pardoned.