Reynoso said the administration seems more attuned to voting rights complaints of African-Americans.
He said the administration also appears reluctant to pursue a complaint against a jurisdiction that is dominated by Obama's fellow Democrats.
After a U.S. Supreme Court decision in June that rendered useless an important enforcement provision of the Voting Rights Act, the administration has focused its voting rights resources on Southern states that are controlled by Republicans.
The Justice Department has initiated or joined suits targeting voter identification laws and redistricting plans in North Carolina and in Texas.
The Justice Department acknowledges it is looking at the situation in Los Angeles, but otherwise declined comment.
Matt Barreto, a political science professor and voting rights expert at the University of Washington, said the evidence against the county is overwhelming and includes a history of racially polarized voting that has hurt Latinos.
"My perspective is that this is one of the easiest cases to be made nationally," said Barreto, who has worked for the group of Latinos that includes Reynoso.
A voter-approved independent board draws California's congressional and legislative districts, but counties retain the authority to devise their own districts.
In Los Angeles, each of five supervisors represents nearly 2 million people, and the county's annual budget tops $26 billion.
Gloria Molina, the only Latina ever elected to the board, and Mark Ridley-Thomas, the board's lone African-American member, had supported maps that would have created a second district with a majority of Latino residents. But the two members could not persuade their three white colleagues to join them.
Molina was elected after a federal court documented political discrimination against Latinos dating back to the 1950s and drew a map to ensure Latinos would be represented.
