Not all US presidential elections are the same, in that sometimes a sitting president is up for reelection, so the governing party simply puts up the president as its nominee and waits for the other party to nominate a candidate. Even if a president is in a weakened political position, rarely do candidates inside the governing party mount a movement to remove him.
Hillary Clinton and Bernie Sanders argue during the Democratic presidential primary debate at the University of Michigan-Flint on March 6. (AP)
Republican U.S. presidential candidates Donald Trump (L) and Ted Cruz speak simultaneously as they discuss an issue during the debate sponsored by CNN for the 2016 Republican U.S. presidential candidates in Houston, Texas, February 25, 2016. REUTERS
Republican U.S. presidential candidate Governor John Kasich speaks during the Republican U.S. presidential candidates debate sponsored by ABC News at Saint Anselm College in Manchester, New Hampshire February 6, 2016