WASHINGTON (Reuters) - The staff of the U.S. Senate -- the advisers and assistants who help keep elected lawmakers offices' humming -- trails the nation in racial diversity, according to a report by a Washington-based research group released on Wednesday.

People of color make up just 21.4% of Senate staff, well below their 41.1% share of the overall U.S. population, the report by the Joint Center for Political and Economic Studies found.

Its analysis focused on what the researchers called "pathway staff," workers in mid-level positions that can lead to senior jobs such as chief of staff or legislative or communication director, and the report noted that a lack of diversity in junior roles can prolong a lack of diversity in those more senior jobs.

"The Senate should reflect the diversity within America," said LaShonda Brenson, senior researcher at the center, which describes itself as "America's Black Think Tank." "That, in turn, will reflect more policies that are more inclusive of all Americans."

People of color made up an even smaller proportion of senior staff roles in Senate offices -- 15.8%, though that was up from their 7.1% share in a similar 2015 study by the group.

Both Democratic and Republican staff diversity lagged the national average, though the gap was wider in Republican offices, the report found.

The 100-member Senate itself trails the U.S. in diversity, counting just four Black members -- Democrats Cory Booker of New Jersey, Raphael Warnock of Georgia and Laphonza Butler of California and Republican Tim Scott of South Carolina. Those four senators' offices collectively employed 21.1% of Black pathway staffers.

Federal law obligates employers with over 100 workers to report their employees' race, gender and other demographics to the U.S. Equal Employment Opportunity Commission, though that requirement does not apply to Congress.

The researchers said they collected their data through an online search of Senate staff, which they then submitted to individual Senate offices for fact-checking.

(Reporting by Allende Miglietta; Editing by Scott Malone and Josie Kao)

By Allende Miglietta