UNIVERSAL CITY, Calif. — Senator Bernie Sanders plans to lay off at least half of his campaign staff Wednesday as his battered presidential bid continues despite Hillary Clinton’s being declared the presumptive Democratic nominee, two people close to the campaign said Tuesday.

Many of those being laid off are advance staff members who often help with campaign logistics, as well as field staff members who have been working to garner votes for the senator, according to a campaign official and a former campaign staff member, both of whom spoke on condition of anonymity. Some campaign workers may move into jobs at Mr. Sanders’s Senate office, but others will be terminated, they said.

Word of the layoffs came on a night that Mrs. Clinton declared that she had captured the majority of pledged delegates needed to capture the Democratic nomination, despite a spirited fight from Mr. Sanders, who has showed no signs of ending his campaign.

Mr. Sanders insists that he is prepared to challenge Mrs. Clinton at the Democratic National Convention in July, holding out hope that his lobbying of superdelegates — party officials and state leaders who cast their final votes at the convention — will siphon support from Mrs. Clinton as he makes his case that he is a stronger candidate against Donald J. Trump, the presumptive Republican nominee.