During presidential elections in the United States, it has become customary for the main candidates (almost always the candidates of the two largest parties, currently the Democratic Party and the Republican Party) to engage in a debate. The topics discussed in the debate are often the most controversial issues of the time, and arguably elections have been nearly decided by these debates (e.g., Nixon vs. Kennedy). While debates aren't constitutionally mandated, it is often considered a de facto election process. The main target for these debates are undecided voters; those who usually aren't partial to either political ideology or party.
Presidential debates are held late in the election cycle, after the political parties have nominated their candidates. The candidates meet in a large hall, often at a university, before an audience of citizens. The formats of the debates have varied, with questions sometimes posed from one or more journalist moderators and in other cases members of the audience. Between 1988 and 2000, the formats have been governed in detail by secret memoranda of understanding (MOU) between the two major candidates; an MOU for 2004 was also negotiated, but unlike the earlier agreements it was jointly released by the two candidates.
Debates are broadcast live on television and radio. The first debate for the 1960 election drew over 66 million viewers out of a population of 179 million, making it one of the most-watched broadcasts in U.S. television history. The 1980 debates drew 80 million viewers out of a 226 million. By 2000, about 46 million viewers out of a population of 280 million watched the first debate, with ten million fewer watching the subsequent debates that year. In 2004, 62.5 million people watched the first debate, while 43.6 million watched the vice-presidential debate.