A presidential by-election was held on January 20, 2015, following the death in office of President Michael Sata. The election was tightly contested between the ruling Patriotic Front (PF) and opposition United Party for National Development (UPND), the parties that eventually finished first and runner-up, respectively. With a charged atmosphere and violence seemingly imminent, Justice Mambilima took a firm stand and warned the two front-runners against trying to intimidate electoral staff. Justice Mambilima, who was sometimes called Zambia's 'iron lady' in reference to her firmness, also warned against claiming electoral victory before the official announcement and inciting premature celebration among supporters. The UPND wrote to Justice Mambilima claiming electoral malpractice by the PF and asking her not to declare a winner, a request the Commission did not indulge. When Edgar Lungu was declared winner, the UPND accused the Electoral Commission of conniving with the PF to manipulate the results, an allegation that Justice Mambilima dismissed. She argued that the process had been transparent and the UPND had been represented at every stage and would therefore also be party to the rigging they were accusing the Commission of. The following month, the UPND supported Justice Mambilima's ratification as Chief Justice.