One hundred days from now, Americans will go to the polls to choose between two candidates who would have been inconceivable for almost all of the nation's history, Democrat Hillary Clinton and Republican Donald Trump. The reason Ms. Clinton would have been inconceivable is a simple one: The former senator and secretary of state is a woman.

Why would Mr. Trump have been inconceivable? Where to begin?

Start with his utter lack of experience in any position of public trust, civilian or military. It is completely unprecedented for a major party presidential nominee. Add his praise for foreign dictators, his idea to pay less than the full value of the nation's debts and his willingness to renege on our bedrock military alliance, and you have a set of ideas with no place in our history. He has proposed religious tests for immigrants and appeared to openly invite a foreign government to commit espionage with the purpose of influencing the presidential election. (Have no fear, though, he was being “sarcastic” about that last one.)

Yet so powerful is the desire among the electorate for an outsider to break up the status quo that none of those things seem to matter. Mr. Trump came out of the Republican National Convention in Cleveland tied or ahead of Ms. Clinton in most polls, and even assuming she sees an uptick in support after the Democrats' convention in Philadelphia, this race is expected to remain close. Mr. Trump remains the undisputed master of exploiting a fragmented media landscape, and his contempt for the conventions of American electoral politics has proved an asset for a public convinced — with no small justification — that Washington is broken.

Ms. Clinton has ardent supporters, but she is viewed unfavorably by a majority of voters. The same, of course, is true of Mr. Trump, but she fares far worse than he does on questions of honesty and trustworthiness, the diligent efforts of fact-checkers to debunk his wild prevarications notwithstanding. The Democratic convention included various attempts to humanize her, but her big speech on Thursday night included a concession on the likability front. “The truth is, through all these years of public service, the ‘service' part has always come easier to me than the ‘public' part,” she said. “I get it that some people just don't know what to make of me.”

We're not going to see a Nixon-with-Checkers attempt at rebranding over the next few months. Rather, Ms. Clinton is showing every sign of doubling down on the other attributes her surrogates ascribed to her in Philadelphia: toughness, relentlessness and resolve. Nobody was harder on Mr. Trump during the Democratic convention than she was.

The Republican convention presented a decidedly dark, pessimistic, fearful image of the country, and the Democratic convention countered it with messages of hope, optimism and love. If the person at the top of the Democrats' ticket was Mr. Obama — “the man of hope,” in Ms. Clinton's phrase — that might be the dynamic that defines this race. Instead, we can expect to see a fear campaign of a different kind from Ms. Clinton — fear not of immigrants or Muslims but fear of Donald Trump as president. The most memorable part of her speech Thursday wasn't her praise of the American spirit but her crystallization of doubts about her opponent: “Ask yourself: Does Donald Trump have the temperament to be commander-in-chief? Donald Trump can't even handle the rough-and-tumble of a presidential campaign. He loses his cool at the slightest provocation. ... Imagine him in the Oval Office facing a real crisis. A man you can bait with a tweet is not a man we can trust with nuclear weapons.”

Ms. Clinton has never faced an opponent as willing to do or say anything as Mr. Trump, or one who has been so impervious to the normal laws of politics. Then again, Mr. Trump, who got a free ride from his rivals until it was too late during the primary campaign, has never experienced anything quite like the onslaught he's about to face from Ms. Clinton. The result is anyone's guess, but one thing is abundantly clear: It's about to get ugly.