That alternation means winners don't put their opponents in jail. If they do -- or if the opponents fear that they will be jailed -- then the incentive to accept defeat evaporates. Losers in that dire position instead will turn to wide-scale popular resistance or military coups. That's only rational if the losers think they won't be free to run for office again.
Alternation is thus what distinguishes stable democracies from weak or failing ones. Prosecuting opponents is the hallmark of democracy-ending dictatorship. Egypt offers a recent and clear example: Abdel-Fattah El-Sisi has relentlessly prosecuted the elected leaders he displaced in his 2013 coup. No one has any illusion that the Muslim Brotherhood will be back in future elections. And no one doubts that democracy in Egypt is over.
It may seem extreme to say that Trump's promise to prosecute Clinton threatens alternation in the U.S. After all, American democracy is pretty stable. But other presidents have bent over backward to avoid such prosecutions -- even to the point of condoning illegal behavior.
Gerald Ford's pardon of Richard Nixon is a prime example, even though the two were from the same party. Ford judged that the republic wouldn't be well-served by prosecution of a former president. George W. Bush didn't seek to prosecute Bill Clinton for perjury, although legally he might have been able to do so.
And Barack Obama didn't seek to prosecute Bush administration officials for acts that likely counted as torture.