As public support turns against him, Donald Trump is suggesting without evidence that the election could be “rigged” by Democrats using vote fraud. This claim threatens the norms of American elections and could provoke a damaging reaction among his supporters.

Ultimately, democracies depend on losers’ acceptance of the legitimacy of the political process. That’s why the norm of accepting election outcomes among defeated presidential candidates is so important.

Though his political allies protested, for instance, Richard Nixon accepted his narrow defeat against John Kennedy in 1960. Similarly, though Al Gore took the 2000 recount fight to the Supreme Court, he ultimately endorsed George W. Bush as the legitimate winner of the election after the court’s decision to end the Florida recount.

Mr. Gore’s concession statement captured the principle that the system depends on acceptance by the loser — no matter how unfair it may feel. In that statement, he said: “While I strongly disagree with the court’s decision, I accept it. I accept the finality of this outcome.” He added, “For the sake of our unity as a people and the strength of our democracy, I offer my concession.”