WASHINGTON -- Two big and passionate crowds have descended on Washington -- one thrilled by Donald Trump's inauguration, the other appalled.  Never in my lifetime has a new president been anticipated with such raw enthusiasm on one side and such fear and loathing on the other.

Admit it, you have no idea what a Trump administration will actually be like. Neither does Trump, I would wager. He is a 70-year-old business executive and self-promoter extraordinaire whose lifelong working habit is to go to his office, see what opportunities the day presents, and then improvise. He is not going to change.

Americans have elected as president a man who was caught on tape boasting of how he assaults women, kissing them and touching their genitals without invitation, and gets away with it because of his celebrity. It is fitting, then, that the biggest planned protest is Saturday's Women's March on Washington, with scheduled speakers such as Gloria Steinem and Angela Davis. A-listers such as Katy Perry are expected to attend.

The star power at the inauguration itself promised to be of much lower wattage. It's no surprise that a Republican president couldn't get Bruce Springsteen to serenade him, but Trump couldn't even get a Springsteen tribute band to perform.

I feel like I've seen this movie before. This reminds me of the Nixon years, when the "silent majority" felt itself assailed by the counterculture of hippies, minorities and uppity women. This time, though, the Trump loyalists are a clear minority; he lost the popular vote and his approval rating is shockingly low for a new president. And the counterculture is now the mainstream: Some of those marching against Trump this weekend haven't taken to the streets since the days of Vietnam. 

There are differences between Trump and every one of his predecessors, however, making this truly a leap in the dark.

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Trump has no fixed ideology. Once a Democrat, he commandeered the Republican Party the way a bank robber might hijack the nearest car to make his getaway. The GOP is Trump's vehicle, not his cause, and there is a chance that some of his policies -- perhaps even in health care -- will give more heartburn to conservatives than to progressives.

He has no experience in government, the military or any kind of public service. In his whole career, he has worked only for his father and himself. Now he has 320 million bosses, and each of us has the right to tell him what to do. I believe this will be a difficult concept for him to grasp.

Trump lies all the time. All presidents have stretched the truth occasionally, of course, and some of them lied frequently and convincingly. But I cannot recall any other public figure, let alone any president, whose every utterance needed fact-checking the way Trump's words do. In "The Art of the Deal," Trump calls his rhetorical method "truthful hyperbole." But there is no such thing.

The new president remains appallingly ignorant about much of the nation he will lead. He must have learned something on the campaign trail, but he seems unaware that most African-Americans are middle class, or that most Mexican immigrants are hardworking and law-abiding -- or, judging from his Cabinet picks, that most billionaires are as out-of-touch as he is.

And finally, Trump's deep insecurity may be the most frightening thing of all. A leader has to be confident enough to let slights and insults pass; by being big, he makes his adversaries look small. Trump has what can only be called a pathological need to respond to any criticism with overkill -- and if you try to swat flies with a sledgehammer, a lot of the furniture gets broken. When he is flailing away at someone like Rep. John Lewis, the civil rights icon, Trump risks only his dignity. But what happens if he feels dissed by someone like President Xi Jinping of China, or by Trump's Russian soul mate Vladimir Putin? The risk is to all of us, and it is incalculable.

So I can't pretend this is a normal inauguration. Of course I celebrate the peaceful and orderly transfer of power, but I also hope that Saturday's protest march is big and loud and spirited -- and that it represents the start of something, not the culmination.

President Trump's power is not unchecked. We, the citizens, are the ultimate authority. We must let him know, through our elected officials and with our own rude voices, when he threatens to go too far.

Get ready. We have work to do.

Eugene Robinson's email address is eugenerobinson@washpost.com.