Republicans, who began attacking the administration’s move hours before Mr. Obama officially announced it, argued that it was unconstitutional and called it a brazen attempt to undercut the role of the Senate to advise and consent the executive branch on appointments.

“This is an extraordinary and entirely unprecedented power grab by President Obama that defies centuries of practice and the legal advice of his own Justice Department,” the House speaker, John A. Boehner, said in a prepared statement. “The precedent that would be set by this cavalier action would have a devastating effect on the checks and balances that are enshrined in our Constitution.”

Republicans had attempted to block the president from installing nominees they opposed over the long break by holding “pro forma” sessions over the holidays, in which members of both chambers — usually those who live in states close to the Capitol — gavel in for minutes or even seconds before gaveling out to try to meet the definition of holding a Congressional meeting. Senate Democrats had taken similar steps to block President George W. Bush from making what are known as recess appointments, which can last up to two years. Until Wednesday, the tactic had prevented two administrations from putting their choices in place without Senate confirmation.

The action by Mr. Obama immediately sparked questions of what constitutes an actual Congressional recess, the legality of Mr. Obama’s move and the future of the Congressional confirmation process.

“The legislative branch exists as a check and a balance on the executive,” Senator Orrin Hatch, Republican of Utah, said in a statement. “By opening this door, the White House is saying it can appoint any person at any time to any position it chooses without the advice and consent of the Senate. This is not how our republic was designed to function.”

But following an internal review, the administration legal team decided that Congress was effectively in recess even though it was holding the pro forma sessions at which no business was conducted.

“Can the Senate, through form, render a constitutional power of the executive obsolete? Our view is that the answer to that question is clearly no,” Kathryn Ruemmler, Mr. Obama’s White House counsel, said in an interview.