History has taught us that disputed Presidential elections not only have damaging short-term effects—they can also have far-reaching consequences for decades after. The most recent experience was in the Bush-Gore election of 2000. The outcome hinged on the final tally in Florida—amid 'hanging chads' and contested counts. Just 537 votes (out of 5.8 million) ended up delivering Florida's 25 Electoral College votes to George W. Bush (after a brief detour to the Supreme Court.). Nobody can know for certain how a 'President Al Gore' would have reacted to the 9/11 terror attacks—or how he would have handled the subsequent Iraq conflict. But it's safe to say that history would have played out at least a little differently.
As disruptive as the 2000 election was, it couldn't hold a candle to the short and long-term impacts of the 1876 election. Eleven years after the end of the Civil War, the nation was still sharply divided—and it showed in the presidential election results. Democrat Samuel Tilden won the popular vote, but fell one vote shy of the majority needed in the Electoral College. Because of disputed delegations in Louisiana, South Carolina and Florida, Republican Rutherford B. Hayes also could not gain an Electoral College majority. So, after much wrangling (that lasted well into the new year), a "deal" was struck—the infamous Compromise of 1877. Democrats agreed to concede the election to Hayes, in return for an end to Reconstruction and the withdrawal of federal troops from the Southern states. The long-term effect was the rise of 'Jim Crow' laws that effectively disenfranchised African-Americans across the South for almost a century afterward.
A disputed 2020 election is unlikely to be as historically impactful as the 1876 contest, but it has the potential to be highly disruptive to the nation.