Those who think about politics and history as a profession can’t resist comparing presidential years. “This is 1968 all over again.” “We’re back to the dynamics of ’72.” We do this because we know political history and love it, and because there are always parallels and lessons to be learned.

But it should be said as a reminder: This year isn’t like any previous time.

This is the year of the sudden, historically disastrous debate, the near-assassination of one of the nominees, the sudden removal of the president from his ticket, the sudden elevation of a vice president her own party had judged a liability, and her suddenly pulling even in a suddenly truncated campaign.

We have never had this year. And it continues to astound.

Kamala Harris just got two excellent weeks in the clear. Donald Trump ’s campaign had to take her down early or at least hit her hard—and didn’t. She has the wind at her back; he’s scattered and stuck on the back of his heels. This week she had a good rally in Atlanta; he went before a hostile National Association of Black Journalists, was taken aback by his first questioner’s accusatory tone, matched her energy, and revealed, if you didn’t know, how cutting and personal the coming months will be.

What is remarkable is how surprised the Trump campaign seems to have been by Ms. Harris. Why? Smart people understood Joe Biden would eventually have to step aside, and she was his most likely replacement. Why have they responded as if shocked? We have a trough of videos of her talking, it’s devastating. Where is it? Is that all you’ll need to make a coherent case? When are you going to locate the meaning of this thing?

“San Francisco liberal,” “way too radical.” All that feels tired, the reflex of an aged muscle. It sounds like the 1990s. This isn’t the ’90s. New ages need new arguments, or at least arguments freshly cast.

Can Mr. Trump shift gears? He grew up, as I did, watching “The Ed Sullivan Show.” I’m sure it was on every Sunday night at 8 at the Trump house in Queens. On that show you saw every week the great Borscht Belt comics of 1950-70. Their timing—“Take my wife—please!”—is ingrained in him. What he does now is shtick, because he likes to entertain and is a performer. The boat’s sinking, the battery’s spitting, the shark’s coming! As Hannibal Lecter said, “I’d love to have you for dinner!”

This works so perfectly for those who support him. For everyone else it’s just more evidence of psychopathology. He has to freshen up his act. Can he?

Ms. Harris will dominate the coming week with the unveiling of her vice-presidential choice. Then there will be the convention, in which they’ll pull out all stops. And then August will be over. Meaning a third of the 100-day campaign will be over. Does Mr. Trump know that he’s fighting for his life?

I want to take a quick look at some factors that are major pluses for Ms. Harris.

• She is new. She seems a turning of the page away from Old Old Biden and Old Old Trump. She looks new, like a new era. She displays vigor and the joy of the battle. The mainstream media is on her side. Coverage hasn’t been tough or demanding.

• On policy she is bold to the point of shameless. This week she essentially said: You know those policies I stood for that you don’t like? I changed my mind! Her campaign began blithely disavowing previous stands, with no explanation. From the New York Times ’s Reid Epstein: “The Harris campaign announced on Friday that the vice president no longer wanted to ban fracking, a significant shift from where she stood four years ago.” Campaign officials said she also now supports “increased funding for border enforcement; no longer supported a single-payer health insurance program; and echoed Mr. Biden’s call for banning assault weapons but not a requirement to sell them to the federal government.” It’s remarkable, she’s getting away with it, and it’s no doubt just the beginning. It will make it harder for the Trump campaign with its devastating videos.

Will the left of her party let her tack toward moderation? Yes. She’s what they’ve got, and in any case people on the wings of both parties have a way of recognizing their own. Progressives aren’t protesting her new stands: That’s the dog that didn’t bark.

• She too is a born performer. She knows what she’s doing when she’s campaigning. She is less sure of what she’s doing when she’s governing. But she gets a race. Running for the 2020 Democratic nomination, she wasn’t good at strategy or policy, but the part involving performing and being a public person and speaking with merry conviction—she gets that and is good at it.

• She is beautiful. You can’t take a bad picture of her. Her beauty, plus the social warmth that all who have known her over the years speak of, combines to produce: radiance. It is foolish to make believe this doesn’t matter. Politicians themselves are certain it matters, which is why so many in that male-dominated profession have taken to Botox, fillers, dermabrasion, face lifts, all the cosmetic things. Because they’re in a cosmetic profession.

• She has a wave of pent-up support behind her. By November we’ll know if something big happened. Barack Obama deliberately, painstakingly put new constituencies together. He created a movement . It had fervor and energy. What we may see this year is something different—that a movement created Kamala Harris. That is, the old constituencies held, maintained fervor and rose again when Mr. Biden stepped aside and Ms. Harris was put on top. I’m not sure we’ve seen that before.

She has many particular challenges. One is this: When you see Mr. Trump, that’s Trump. He is what you think he is. He doesn’t hide much. You look at him and think (pro or con), OK, I get it, I know who that guy is. When you see Ms. Harris, is that Harris? Is what she is showing you her? You wonder, “Is this real and genuine?” I wonder how she’ll address that or answer it.

Another: She stumbles in interviews. Will she try to get away with not doing any?

Another: People will continue to wonder how liberal she is, and how strong she is, but I think an equally or more important question will be how serious she is. Does she think seriously, deeply, soberly? I haven’t seen her betray this tendency. Mr. Obama was a serious man, Hillary Clinton was fully understood as a serious woman. (That’s why her campaign could produce and she could capitalize on the famous “3 a.m. phone call” ad.) Is Ms. Harris? Is she a credible commander of the U.S. nuclear arsenal?

Some will respond, “But Donald Trump isn’t serious!” My answer would be: That’s why he lost the popular vote twice. If Democrats lose the popular vote, they almost certainly lose the election.

Mr. Trump himself would reply: I controlled the nuclear arsenal for four years. Nothing blew up.