The historic appointment of Lonnie G. Bunch III as secretary of the Smithsonian Institution creates a leadership vacuum at the National Museum of African American History and Culture, where he has been founding director since 2005.

Celebrated for its powerful exhibitions and bold programs, the museum has become the Smithsonian’s crown jewel, thanks in large part to Bunch’s vision and leadership.

He will be a difficult act to follow.

Enter Spencer Crew — a Smithsonian insider, guest curator of one of the African American Museum’s inaugural exhibitions and a close friend of Bunch. Crew will step in as interim director this summer as Smithsonian officials begin a national search for Bunch’s permanent replacement.

Bunch said the museum is lucky to have landed Crew for the temporary post.

“We have someone who can lead the organization, that can help it continue its fundraising, its programmatic initiatives. I’m very comfortable,” Bunch said Tuesday. “Not only is [the museum] in good hands with its leadership, but the staff there, who I hired, is really wonderful and they are able to carry on the vision of the museum.”

The 19th and newest Smithsonian museum opened Sept. 24, 2016, and continues to attract near-capacity crowds. More than 4 million people have visited its wide-ranging exhibitions, which include galleries on slavery, segregation, sports, religion and entertainment.

In addition to curating the exhibit “Defending Freedom, Defining Freedom: Era of Segregation 1876-1968,” Crew spent almost a decade as director of the Smithsonian’s National Museum of American History. He was also president of the National Underground Railroad Freedom Center for six years. He is a history professor at George Mason University and has written several books.

Bunch and Crew have worked together for 20 years. At times Crew has been the boss, at other times Bunch.

“It’s quite a task to take on and quite a task to follow someone like Lonnie Bunch,” Crew said Wednesday. “It’s not something that you can turn down.”

The museum’s advisory council is focused on finding an outstanding successor, said Kenneth Chenault, the panel’s chairman. The council has discussed succession plans in recent years, knowing there was a chance that Bunch would leave.

“Lonnie gave 14 years. He has built an incredible foundation,” Chenault said. “We want someone just like [him], who was transforming, who could inspire the really terrific people we have at the museum to reach new heights, someone who has credibility in the academic community as well as someone who is a strong leader.”

Chenault would not share details about the search process or how long it would take. Bunch’s first day as secretary is June 16, but Crew’s exact start date had not been set. The museum will maintain its momentum with Crew, Chenault said.

“Spencer understands the Smithsonian, he was heavily involved in the inaugural exhibitions . . . and he is one of the foremost historians of African American history,” he said.

Crew said his focus will be on supporting the staff and helping the museum stay the course.

“It is important to me,” he said. “I certainly want to do whatever I can to help it to continue to be the kind of place people want to come to.”

The mood at the museum is bittersweet, said Tasha Coleman, assistant director of council operations and special initiatives. Coleman, who has worked with Bunch since the day he arrived, described Tuesday’s staff meeting as “jubilant and emotional.”

“Everyone was celebrating this accomplishment. There was so much happiness and joy for him,” she said. “There’s the comfort of knowing he’s not gone. He’s now the secretary, and he’s going to be looking out for us.”

Coleman also said that the staff has always known Bunch could leave.

“He’s done an expert job making the vision a part of our DNA,” she said. “People are committed to the vision, not necessarily to the man. His presence in the museum will be missed, but knowing this is the next step, the greater purpose trumps ‘I’m sad to see you go.’ ”