Mr. Booker’s campaign still commands a staff of about 50 in Iowa, many of whom have been working in the state since the summer, as well as roughly 30 staff members in New Hampshire and 20 in both Nevada and South Carolina.

The campaign has put some of its influx of cash toward airing its first television ad in Iowa, spending more than $300,000 so far, with the goal of boosting Mr. Booker’s poll numbers in the state to help qualify him for the January debate.

“We’re thrilled to have closed out 2019 with our best fund-raising quarter yet, despite not being able to join the December debate stage due to the artificial thresholds that prevented viable candidates from participating,” Addisu Demissie, Mr. Booker’s campaign manager, said in a statement. “But to put it bluntly, we’re still behind the fund-raising of a field of predominantly white candidates who have been able to haul in significantly greater sums of money or tap into their personal fortunes to fund their campaigns.”

Reid J. Epstein reported from Hanover, and Nick Corasaniti from New York.