“If in a few months, we look back and see that this Trump strategy was just an utter failure, then it’s not likely to be copied,” said Mr. Ginsberg, who represented former President George W. Bush in the 2000 election standoff. “But the system was stress-tested as never before.”

That test, he said, revealed enough vague provisions and holes in American election law to make a crisis all too plausible. He pointed in particular to the lack of uniform standards for the timely certification of elections by state authorities, and the uncertainty about whether state legislatures had the power to appoint their own electors in defiance of the popular vote. The 2020 election, he said, “should be a call for some consideration of those issues.”

Yet even without precipitating a full-blown constitutional crisis, Mr. Trump has already shattered the longstanding norm that a defeated candidate should concede quickly and gracefully and avoid contesting the results for no good reason. He and his allies also rejected the longstanding convention that the news media should declare a winner, and instead exploited the fragmentation of the media and the rise of platforms like Twitter and Facebook to encourage an alternative-reality experience for his supporters.