Former South Bend, Indiana, Mayor Pete Buttigieg s presidential campaign filled less than half of the delegate slots for the four most diverse congressional districts in Illinois, a sign of how his campaign continues to struggle with Black and Latino voters.

Voters in Illinois cast ballots not only for their pick for president, but also for delegates to the Democratic National Convention. Delegates are generally associated with one of the presidential campaigns, meaning each campaign needs to find 101 people willing to serve as a delegate at least three in each of the states 18 congressional districts, with additional delegates in more heavily Democratic districts.

Finding a person to fill each slot and collecting the 500 signatures necessary to put them on the ballot is considered an early test of a campaigns strength in the delegate-rich state, and supporters of Buttigiegs rivals said his failure to fill the slots points to a potentially fatal weakness with voters of color.

The three leading Democratic candidates in national polling all filled an overwhelming majority of slots. Massachusetts Sen. Elizabeth Warren and Vermont Sen. Bernie Sanders filled every available slot, while former Vice President Joe Biden filled all but three slots. Buttigieg, who trails the other three candidates in national polling but is a top contender in the early voting state of Iowa, filled just 53 of the slots. Two lower-profile candidates, Minnesota Sen. Amy Klobuchar and entrepreneur Andrew Yang, both filled more than 60 slots.

Buttigiegs missing delegates are concentrated in the states most diverse congressional districts, according to an analysis of candidate data files from the Illinois State Board of Elections. Illinois has two majority-Black districts: The 1st, represented by Rep. Bobby Rush, and the 2nd, represented by Rep. Robin Kelly. Buttigieg filled zero of the eight slots in the 1st District, and just four of seven slots in the 2nd District.