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). Candidate debates are not constitutionally mandated, but it is now considered a de facto election process. The debates are targeted mainly at undecided voters; those who tend not to be partial to any political ideology or party.