WASHINGTON — Vice President Kamala Harris is zeroing in on four potential candidates as she races to choose a running mate for her fledgling campaign, fast-tracking a process that usually takes months but must be finalized in only a few weeks. Eric Holder, the former U.S. attorney general, and a team of lawyers at his firm Covington & Burling are taking the lead on vetting potential choices, according to two people with knowledge of the matter who requested anonymity because the process is being closely held. The political conversation has centered on an assortment of white men — Arizona Sen.