"It was abundantly clear we didn't have air cover," Priebus said in a statement, referring to support in the form of television advertisements. "No one can fix this problem better than Katie Walsh."
White House officials argued that Walsh's departure should not be considered part of a shake-up, saying that she leaves with the blessings of Trump's top advisers and with the explicit aim to fill an important role outside the government, shoring up political support for Trump.
"Katie Walsh was instrumental in the victory in November," Jared Kushner, the president's son-in-law and a senior White House adviser, said in a statement. "There is no one better suited to fulfill this role than Katie."
Walsh said in a statement: "It has been the honor of a lifetime to work for President Trump in the transition and in the White House. I am excited for this new opportunity and the ability to continue to fight for the president's agenda to 'make America great again.'"
Walsh's exit was first reported by Politico.
Walsh was one of the few women holding a senior role in the Trump White House and was seen as a strong force for organization, although some Trump loyalists viewed her with suspicion because of her roots in the Republican establishment. Walsh previously worked as chief of staff at the Republican National Committee, under Priebus as chairman, and before that as the committee's finance director.
"Katie's a vital link that pulls things together and makes things happen," Stephen K. Bannon, the chief White House strategist, said in a statement.
Walsh will work with America First Policies, one of several pro-Trump outside groups, according to her fiance, Mike Shields, a fellow Republican operative. He tweeted that Walsh had been asked by some of Trump's top advisers to provide "badly needed air cover for the President's agenda."
Walsh’s move is a sign of the White House’s deepening dissatisfaction with the state of President Trump’s allied outside operation. The advocacy group she plans to advise was rolled out back in January as a robust flanking effort that would be led by a team of former Trump campaign and transition officials. But the group has moved slowly, playing little role in Trump’s recent push to replace Obamacare.
One of the group’s leaders, Rick Gates, recently stepped down amid new scrutiny of the ties between his longtime business associate Paul Manafort and Russian business interests. Another, former deputy campaign manager David Bossie, left to launch another pro-Trump group called Making America Great, backed by GOP mega-donor Rebekah Mercer. That organization has already rolled out a $1 million TV ad buy pressuring Democratic senators to back Trump.
Still attached to America First Policies is Nick Ayers, who served as a top adviser to Vice President Pence; former Pence aide Marty Obst; and Brad Parscale, who served as Trump's digital director. Walsh’s arrival could set up an intensifying competition between the organization and the Mercer-backed group.