The Bernie Sanders campaign is laying off hundreds of field staffers after a tough stretch in which he lost five of the last six nominating contests and now has virtually no chance of mounting a comeback to wrest the Democratic nomination from Hillary Clinton.

Though Mr. Sanders has surpassed fundraising expectations, the campaign doesn’t have “unlimited resources” and cannot afford to keep on the payroll a full complement of field staff with three-quarters of the Democratic contests completed, a person familiar with the matter...