Republican senators on Wednesday joined a lawsuit arguing President Obama violated the Constitution when he used his recess appointment powers earlier this year to fill several controversial posts.

The move intensifies a simmering separation-of-powers battle over Mr. Obama’s executive powers, which Republicans say he has abused during his term in office.

Led by Senate Minority Leader Mitch McConnell of Kentucky, the Republicans said the Senate was still in session, not in recess, when Mr. Obama made his four appointments in January. They said the picks — to the National Labor Relations Board and to a top consumer-advocacy post — are therefore illegal.

“The Constitution does not permit the president to bypass the Senate whenever he deems it uncooperative,” the Republicans said in their legal brief. “The Senate’s veto over appointments provides a vital check … Allowing the president to invoke the [appointment] clause to end-run Senate opposition to his nominees eliminates that check, severely undermining the separation of powers.”

Republicans had signaled their intent to block the consumer advocacy and NLRB nominations, hoping to hinder Mr. Obama’s agenda by refusing to fill those posts.

But with senators at home for Christmas and New Year’s, and the chamber only holding pro forma sessions, Mr. Obama decided they were in a recess and said that meant he could use his emergency appointment powers.

The senators are joining an already existing lawsuit filed by Noel Canning, a bottling company that sued the NLRB, arguing that a decision the board made is invalid because Mr. Obama’s questionable appointees were part of the board at the time.

The suit was first heard by the NLRB, and is now before the U.S. Circuit Court of Appeals for the District of Columbia. The senators said this is the right time to intervene.

If the appeals court sides with Republicans, it could invalidate a host of decisions made by the board.

The key legal question is whether the Senate was in session and able to give advice and consent on nominations, as the Constitution charges it with doing.

Mr. Obama’s Justice Department in January wrote a legal memo defending the president’s move, saying that the Senate was not in session in any meaningful sense of the word since business was generally prohibited at the sessions they were holding twice a week.

Indeed, usually there was just a single senator present to preside, and he or she would gavel the Senate in and out of session in less than a minute. The vast majority of senators were back in their home states or out on official trips.

“We do not believe that the convening of periodic pro forma sessions precludes the president from determining that the Senate is unavailable during an intrasession recess otherwise long enough to support the president’s recess appointment authority,” wrote Virginia A. Seitz, an assistant attorney general, in the memo.

But Republicans, in their legal filing, said sometimes legislation is passed in those pro forma sessions, and said Mr. Obama has signed some of those bills — which they said indicated he accepts that those are legitimate sessions.

Democrats had actually pioneered the technique of pro forma sessions under President George W. Bush, using them to try to deny him the chance to make recess appointments. Mr. Bush never tested his powers.

But this year, after Mr. Obama did use his powers, Democrats reversed course and said he was justified.

Senate Democratic leaders didn’t respond to a request for comment on the GOP’s legal move on Wednesday.

In a curious twist, Republicans have hired Miguel Estrada to be their lawyer. Mr. Estrada was once a nominee for a top judicial post, but Senate Democrats filibustered to block his nomination, making him the first casualty of the partisan judicial nomination wars under Mr. Bush.

No Democrats joined the GOP in the lawsuit, and five Republican senators also didn’t sign on.

Two of them, Sens. Scott P. Brown of Massachusetts and Dean Heller of Nevada, are locked in tight re-election battles back home and have somewhat distanced themselves from their colleagues. Retiring Sen. Olympia J. Snowe, a Maine Republican who sometimes sides with Democrats, didn’t join, nor did Sen. Lisa Murkowski, an Alaska Republican who tussled with her party in the 2010 elections.

Sen. Mark Kirk, Illinois Republican, is still recovering from a stroke and hasn’t been in the Senate chamber for more than a year.

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