No president has campaigned strongly for his chosen successor in at least 100 years.

Tuesday's event, with President Obama campaigning for Hillary Clinton, his former secretary of state and onetime rival, in North Carolina is remarkable for that reason. It kicks off what is likely to be a season of vigorous campaigning by the president.

There are reasons presidents haven't campaigned strongly for a successor — sometimes they're unpopular, some nominees try to distance, some presidents were in failing health.