Some would argue that we saw this before in the 2000 election. But that was a fluke in which Gore won the popular vote but lost the Electoral College vote (that has happened before starting with the 1876 election). He conceded when the Supreme Court effectively elected George W. Bush for us.
What if this year, the whole process is seen to be so rotten that Electoral College vs. popular vote doesn't matter, that there is a cloud over the legitimacy of whoever wins? What if the "loser" doesn't concede, and the race is decided by a very unpopular Congress? Or, what if the "loser" files multiple lawsuits and the outcome drags through multiple court cases leaving anything like a final and legitimate outcome up in the air? What if such a scenario complicates foreign governments recognizing the legitimacy of whoever is elected President? Could we have our own Ahmadinejad problem where both foreign governments and a sizable part of the population refuse to recognize his legitimacy as president?
Drew opposes the idea of amending the Constitution to undo the Citizens United decision. I understand her scruples and her cautions about such an action, but I'm not sure there is much other recourse for undoing a Supreme Court decision. Such decisions are usually undone by other Supreme Court decisions decades later, or by amendments to the Constitution. Plessy vs Ferguson wasn't undone by the Court until almost 6 decades after the decision. The Dred Scott decision was undone only after the Civil War by the 13th and 14th Amendments. As far as I'm concerned, the Citizens United decision is about as bad as those.
The protests and counter-protests filling the streets of Moscow and other major Russian cities over the legitimacy of their elections could be a glimpse into the future for us.
Protesters in Moscow
It doesn't help matters that Republicans, and even more so their corporate and plutocratic patrons, cherish the dream of permanent single party rule. The right never liked this democracy business, especially when the "wrong people" turn out in large numbers to vote.