Trump vs. Clinton: Is a 2017 'peaceful transfer' possible?
The dynamics of the race, more ominous than the usual rough-and-tumble of politics, leave many Republicans and Democrats worried that many voters will be unwilling to accept the outcome.
Intense, sustained opposition diminishes a president's political capital and emboldens opposition lawmakers who have to answer to their own supporters.
"Politics has never been genteel ... but generally both parties and their leaders have recognized the legitimacy of the process, and that seems to be fraying," said Republican Steve Schmidt, top strategist for Arizona Sen. John McCain's 2008 presidential campaign.
The GOP mood was on display at their national convention, where delegates in Cleveland erupted daily into chants of Lock her up!
The 2000 race between Republican George W. Bush and Democrat Al Gore resulted in a prolonged recount of Florida's votes and ended with a 5-4 Supreme Court ruling that put Bush in the White House.
Lehane recalled being on Capitol Hill, preparing for a news conference, when the court issued its ruling that ended Gore's presidential hopes.
Neither campaign responded to an Associated Press inquiry asking whether the candidates would commit now to an unequivocal concession upon defeat.
The next president could run into trouble from an unlikely source — the winning party, adding one more impediment to a governing mandate.
Trump has yet to secure the endorsement of his closest primary rival, Texas Sen. Ted Cruz, and establishment Republicans remain uneasy about their nominee.