“We conclude that while Congress can prevent the president from making any recess appointments by remaining continuously in session and available to receive and act on nominations, it cannot do so by conducting pro forma sessions during a recess,” the head of Justice’s Office of Legal Counsel, Virginia Seitz, wrote in a 23-page opinion.

On Jan. 4, Obama announced four recess appointments, including one for Richard Cordray to head the Consumer Financial Protection Bureau. The president faulted the Senate for failing to act on the nominations and said it was important to get the officials in place.

White House officials argued at the time that the series of brief sessions the Senate held over the Christmas break were not sufficient to block the president’s power to name temporary appointees when Congress is in recess. They said the Justice Department had been consulted but until Thursday had declined to say whether the Office of Legal Counsel issued a formal opinion and refused to make any opinion public.

Seitz’s opinion is dated Jan. 6, two days after Obama announced the appointments of Cordray and three members of the National Labor Relations Board. Officials said that in keeping with common practice, the Justice Department shared its legal conclusions with Obama before he made the appointments, but more time was needed to polish the formal opinion.

The opinion does not portray the case for Obama’s authority as a legal slam dunk. Seitz concluded that arguments that he lacks such power are “substantial.” She also said there is some chance legal challenges to such recess appointments might prevail.

“The question is a novel one, and the substantial arguments on each side create some litigation risk for such appointments,” Seitz wrote.

The general thrust of Seitz’s opinion is that the Senate’s pro forma sessions are no obstacle to a recess appointment because the Senate is not truly open for business during sessions that may last only minutes and can involve only a single senator.

“The text of the Constitution and precedent and practice thereunder support the conclusion that the convening of periodic pro forma sessions in which no business is to be conducted does not have the legal effect of interrupting an intrasession recess otherwise long enough to qualify as a ‘Recess of the Senate’ under the Recess Appointments Clause,” she wrote.

She noted that many members of the Senate have referred to it as being in recess despite the brief sessions, which Republicans have insisted on as a way of blocking just the kind of recess appointments Obama made.

A number of prominent conservatives, including at least eight Republican senators, had called on the Justice Department to make public any legal opinion supporting Obama’s move. Sen. Chuck Grassley (R-Iowa), who circulated the letter, called the Justice Department opinion “unconvincing.”

“Its conclusion is at odds with the text of the Constitution and the administration’s own previous statements. It fundamentally alters the careful separation of powers between the executive and legislative branches that the framers crafted in the Constitution,” Grassley said in a statement Thursday.

“It relies on no Supreme Court decision and many conclusions are unsupported in law or the Constitution. It recognizes that the courts might well disagree. And it flies in the face of more than 90 years of historical practice. Taken together with a laundry list of other assertions of the power to act without Congress, this is clearly an escalation in a pattern of contempt for the elected representatives of the American people.”

Seitz’s opinion acknowledges a key weakness in the administration’s position, which is that the Senate sometimes does significant business during the brief pro forma sessions. That was the case on Dec. 23, when a major bill extending the payroll tax cut was formally approved.

“It could be argued that the experience of recent pro forma sessions suggests that the Senate is in fact available to fulfill its constitutional duties during recesses punctuated by periodic pro forma sessions,” the OLC chief wrote. She noted, however, that the Senate’s scheduling order expressly provided that there was to be “no business conducted” during the pro forma sessions.

“The president may properly rely on the public pronouncements of the Senate that it will not conduct business…regardless of whether the Senate has disregarded its own orders on prior occasions,” Seitz wrote.

Another awkward fact acknowledged in Seitz’s opinion is that less than two years ago the Justice Department sent the Supreme Court a letter-style brief noting that “the Senate may act to foreclose [recess appointments] by declining to recess for more than two or three days at a time over a lengthy period.”

Seitz insisted that the letter, from then-Solicitor General Elena Kagan, didn’t directly address the situation Obama faced earlier this month.

The Office of Legal Counsel is considered the key arbiter of legal questions for the executive branch. Many of the office’s opinions are published in bound volumes and cited as a kind of quasi-precedent by government lawyers and even judges.

Agencies and presidents rarely act contrary to the office’s advice, but they are not technically required to heed the office’s legal interpretations. Last year, Obama chose to continue using Predator drones in Libya even though the Office of Legal Counsel said doing so would likely violate the War Powers Resolution.

Seitz, a law clerk to Justice William Brennan and subsequently a partner at D.C. law firm Sidley Austin, was confirmed last June to head up OLC. The Senate blocked Obama’s first nominee for the post, Dawn Johnsen, for more than a year before she withdrew.