When Nancy Pelosi took impeachment "off the table" during the 2006 campaign, what was her justification? She said the most important reason to elect a Democratic majority was to give them "subpoena power."
Well We the People beat the unbeatable Bush-Rove political juggernaut, elected Democratic majorities, and made Pelosi the Speaker of the House. And then the Democratic chairs of the various committees started issuing subpoenas. And then White House counsel Fred Fielding told the Democrats (politely and very slowly) to go Cheney themselves.
So now Speaker Pelosi has to make a decision - whether to enforce those hard-earned Democratic subpoenas, or simply throw them into the trash can.