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The opinion seems to conflict with one issued by the same office during the Clinton administration, which said as long as Congress was meeting every three days, no recess appointment could be made.

Congress held a pro forma session on Jan. 3, and Mr. Obama made his appointments on Jan. 4.

The Justice Department’s opinion points to a number of public statements by members of Congress saying they are not in session, but grappled with some major sticking points.

For one, the two-month payroll-tax cut bill Mr. Obama signed into law late last month was passed by the Senate in a pro forma session — highlighting the fact that business can occur, when the chamber gets unanimous consent to do so.

The Senate also can receive nominations from the president in pro forma session, though such messages aren’t entered into the record officially until the full Senate reconvenes. Senators cannot introduce legislation without getting unanimous consent from all other members beforehand.

Furthermore, unconfirmed nominations have not been returned to the White House — something Senate rules say must occur if the chamber recesses for more than 30 days. That signals that the Senate does not consider itself in recess right now.

Two years ago, Mr. Obama’s deputy solicitor general, Neal Katyal, told the Supreme Court that the administration accepted the three-day precedent by which other administrations have abided.

Reached by email last week Mr. Katyal, now a professor at Georgetown University, told The Washington Times that “for now at least I am refraining from comment.”

Part of the problem is that the Constitution uses the word “session” to mean different things.

Each two-year Congress is divided into two sessions, and each daily meeting of the House or Senate is considered a session.

Making appointments when Congress has adjourned for the year — what’s known as an intersession recess — has many precedents, including some when that recess was less than three days.

But Mr. Obama acted during an intrasession recess, and for that, there is scant recent precedent.

The issue has popped up on the campaign trail, where Republican candidates have called the mode illegal and urged Congress to pass a law stripping the appointees of power.

Such legislation is unlikely to advance given that Democrats control the Senate and, while most they have been silent on the legality of Mr. Obama’s use of recess powers, still praised the nominees.