Peter M. Rouse, the former White House official who was President Obama’s longest-serving top aide, has landed in the private sector.

Next month, Rouse and Mark A. Patterson, who was chief of staff to the last two Treasury secretaries, will join Perkins Coie LLP, where the two will set up what the law firm is describing as a new public and strategic affairs practice.

They were recruited by longtime Perkins Coie partner Robert Bauer, who was White House counsel to Obama from December 2009 to July 2011.

Associates said that Rouse and Patterson will not engage in lobbying and noted that Perkins Coie does not have a lobbying operation.

Instead, they will provide advice to the firm’s clients on how to navigate complex problems involving the government, public relations and the legal process.

Rouse was one of the first Obama hires in Washington after Obama was elected to the U.S. Senate from Illinois in 2004.

As it happened, Rouse had found himself unexpectedly out of work after his previous boss, Senate Minority Leader Thomas A. Daschle (D-S.D.), was defeated in that Senate election.

On Capitol Hill, Rouse was sometimes called the “101st senator” because of his deep understanding of the institution and his ties to its members.

“When I lost my Senate race, he literally found jobs for every single staff person,” Daschle recalled. “His network is truly amazing.”

Rouse was also known for offering his advice unvarnished — often in lengthy memos — and discreetly.

In 2008, Obama — by then close to wrapping up the Democratic nomination for president — told Time magazine: “My chief of staff in the Senate, Pete Rouse, Tom Daschle’s old chief of staff, is as well connected and well known and as popular and as smart and savvy a person as there is on Capitol Hill. But is completely ego-free.”

Though Rouse, 67, had long talked about moving on from the White House, Obama persuaded him to stay. He finally left at the end of last year.

Patterson, 52, was one of Rouse’s many Capitol Hill proteges, going back to their time working for Daschle.

He joined the Treasury Department in 2009, as chief of staff to then-Secretary Timothy F. Geithner, and remained there through the early months of Jack Lew’s tenure as Geithner’s successor.

Patterson’s initial appointment had generated some controversy, given that he had been a lobbyist for the banking giant Goldman Sachs. By the time he left Treasury last year, however, he had been the longest-serving chief of staff in the department’s history.