The office of Sen.-elect Tammy Duckworth (D-Ill.) said it has hired eight senior-staff hires, including five women and three people of color. (M. Spencer Green/AP)

In December 2015, the Joint Center for Political and Economic Research released a study about diversity on the staffs of the U.S. Senate.

People of color made up just 7.1 percent of all senior-level staff, while they accounted for more than 36 percent of the nation’s population.

One year later, not much has changed. In a letter sent to the six men and women who won Senate seats for the first time in November, the Joint Center and 51 other civil rights, economic and political inclusion organizations, trade groups and think tanks called on the senators-elect to do something about what they see as a huge problem of nearly all-white senior Senate staffing.

Diverse staffs matter because they help to bring a range of perspectives, experiences and concerns to the Senate’s work, said Spencer Overton, president of the Joint Center.

Of the 297 people working in the three senior level senate staff posts tracked by the Joint Center, just 21 — that’s 7 percent — are people of color. The three positions tracked by the Joint Center include chiefs of staff, legislative and communications directors. Right now, just one African American works as chief of staff for Sen. Tim Scott (R-S.C.), and another as communications director for Sen. James Lankford (R-Okla.).

While Scott’s and Lankford’s offices are exceptions, the problem extends to both sides of the aisle. Rafael Collazo, director of political campaigns for the National Council of La Raza, said that certain Senate practices make greater diversity difficult to achieve.

For instance, in most offices Senate internships are unpaid and are, therefore, more likely to go to young people from affluent families. The relationship between race, ethnicity and income remains such that most of the Senate’s interns come from white families with the ability to finance several months of unpaid life in Washington, Collazo said.

From the Joint Center’s Nov. 18 letter:

“Congratulations to you all on your victories. . . . As each of you put together your staff, it is critical to do so with a recognition of the profound lack of racial diversity that currently exists among staff in the U.S. Senate, and to take an approach that embraces inclusiveness …

As a new Member of the U.S. Senate, you have a unique opportunity to dramatically increase diversity among top staff. Hiring just one African American top staffer, for example, would increase African American personal office top staff representation by 50 percent over 2015 levels. Many of you enjoy significant racial diversity in your states (chart below). Ensuring racial diversity among your top staffers in Washington, DC would enhance the deliberation, innovation, legitimacy, and outcomes of your office and of the Senate as a whole.”

When The Washington Post reached out to the six Senators-elect — Catherine Cortez Masto (D-Nevada), Tammy Duckworth (D-Ill.), Kamala Harris (D-Calif.), Maggie Hassan (D-N.H.), Chris Van Hollen (D-Md.) and Todd C. Young (R-Ind.) — representatives with Cortez Masto and Young’s offices did not respond to a request for comment by deadline.

The remaining four said that staff diversity is a priority. But they measure diversity beyond the three positions identified by the Joint Center.

On Friday, Senator-elect Duckworth’s office announced what it described as eight senior-staff hires, including five women and three people of color.

By Monday, Van Hollen’s office told the Washington Post that senior Senate office staff have not yet been hired. But, Van Hollen’s existing House staff is 60 percent female, 50 percent people of color, and 20 percent LGBT. All of these workers will be joining Van Hollen’s senate staff.

Also on Monday, Harris announced two hires her office described as senior members of the team including a white male chief of staff, Nathan Barankin, Harris’s deputy during her time as California’s attorney general, and a Latina state office director, Julie Chavez Rodriguez.

Barankin managed the California attorney general’s 5,000 employees, worked as a top litigator during Gov. Jerry Brown’s tenure as attorney general, served as the agency’s spokesman and a senior adviser to members of the California Senate’s leadership. Chavez, who is the granddaughter of legendary activist Cesar Chavez, has been a special adviser in the Obama White House where she worked on immigration, health care reform and veterans issues.

Hiring continues in all four offices.