The three-judge panel flatly rejected arguments from the Justice Department's Office of Legal Counsel, which claimed that the president has discretion to decide that the Senate is unavailable to perform its advice and consent function.
"Allowing the president to define the scope of his own appointment power would eviscerate the Constitution's separation of powers," Chief Judge David Sentelle wrote in the 46-page ruling. He was appointed by President Ronald Reagan.
The court ruled that during one of those pro forma sessions on Jan. 3, 2012, the Senate officially convened its second session of the 112th Congress, as required by the Constitution.
Sentelle's opinion was joined by Judge Thomas Griffith, appointed to the court by President George W. Bush, and Karen LeCraft Henderson, who was appointed by President George H.W. Bush.
"With this ruling, the D.C. Circuit has soundly rejected the Obama administration's flimsy interpretation of the law, and (it) will go a long way toward restoring the constitutional separation of powers," said Sen. Orrin Hatch, R-Utah.
GOP House Speaker John Boehner welcomed the ruling as "a victory for accountability in government."
If the ruling stands, it would invalidate more than 600 board decisions issued over the past year. It also would leave the five-member labor board with just one validly appointed member, effectively shutting it down. The board is allowed to issue decisions only when it has at least three sitting members.
Obama used the recess appointment to install Deputy Labor Secretary Sharon Block, union lawyer Richard Griffin and NLRB counsel Terence Flynn to fill vacancies on the labor board, giving it a full contingent for the first time in more than a year. Block and Griffin are Democrats, while Flynn is a Republican. Flynn stepped down from the board last year.