I hate to be an alarmist, but Donald Trump could be on course to be elected president of the United States – and the decisive moment may well come on Monday night. That’s when he faces Hillary Clinton in what is expected to be one of the most watched events in television history. The TV debates are perhaps the last chance for her to persuade the American people that this man is unqualified for, and unworthy of, the presidency and poses a genuine threat to the republic.

If that sounds like panic, then I’m not the only one. Sweaty-palmed nausea has become a leading symptom among those who tremble at the prospect of a Trump presidency. The latest Slate Political Gabfest podcast is called The Time to Panic Edition. Number cruncher Nate Silver, who gives Trump a 40% probability of winning, triggered another round of liberal angst this week when he tweeted that he had “Never seen otherwise smart people in so much denial about something as they are about Trump’s chances. Same mistake as primaries, Brexit.”

Never seen otherwise smart people in so much denial as they are about Trump’s chances. Same mistake as Brexit Nate Silver

The source of the alarm is not so much the national polls, where Clinton is a few points ahead, but surveys from those battleground states where the presidency will be decided. In the last week, polls have put Trump in front in Ohio, Nevada, North Carolina and, most neuralgic of all, Florida.

So low are expectations for his performance on Monday – where it is assumed that his opponent, a seasoned debater, will wipe the floor with him – that if Trump manages to speak in vaguely coherent sentences and not deliver a misogynist insult to Clinton’s face, his advocates will declare that he looked “presidential” and anoint him the winner. If he can somehow persuade wavering voters that he is not so ridiculous as to merit automatic disqualification, he will have cleared a crucial hurdle.

And for all her experience, Clinton heads towards this first, and therefore most important, debate facing some serious obstacles. She’s been advised that she mustn’t interrupt too much or talk over Trump: apparently voters react badly to seeing a woman act that way. According to a New York Times report well sourced from inside Hillary’s debate preparation team, “she does not want to be seen as pushy and play into gender stereotypes”. This was not something Barack Obama, or husband Bill, ever had to worry about.

Facebook Twitter Pinterest Activists in Berlin dismantle what they called a Wall of Hate showing Donald Trump in front of the Brandenburg Gate on 23 September. Photograph: Sean Gallup/Getty Images

But Trump himself presents her with an unprecedented conundrum. How does she take on a candidate who lies and lies and lies? One reporter following Trump has taken to logging the falsehoods he tells each day: no small task.

Take two areas where you’d assume that to be caught in untruth would be terminal for a presidential candidate. Trump has been exposed for serial lying over 9/11, falsely claiming to have lost hundreds of friends that day; to have seen thousands of Muslims cheering the fall of the twin towers; and to have helped in the rescue effort. He even milked cash from a 9/11 relief fund.

This week he was found to have raided $258,000 from the charitable foundation that bears his name to settle assorted legal problems. Earlier he dipped into that same charity box, filled by others’ donations, to pay for a larger than life $10,000 painting – of himself. He lies about 9/11. He lies about charity. But on he strides.

Facebook Twitter Pinterest “But Trump himself presents her with an unprecedented conundrum. How does she take on a candidate who lies and lies and lies?’ Photograph: Evan Vucci/AP

So where can Hillary begin? If she calls him a liar, he’ll hit back over her use of a private email server, calling her Crooked Hillary, knowing that some 60% of Americans believe she’s dishonest. The risk is that he’ll use her experience and expertise against her, casting her as the boring know-it-all against the freewheeling, entertaining guy who relies on good old gut instinct.

If that sounds familiar, that’s because it’s a show we’ve already seen. In 2000 Al Gore was clearly the more accomplished, ready-for-office candidate. But he came across as impatient and pompous against the looser, backslapping George W Bush. Never mind that one was a visionary on the issue of climate change while the other would lead the US into the catastrophic invasion of Iraq. On the night, Bush came across better on TV.

Which brings us to a core problem facing Hillary Clinton. Polls show there are millions of voters, especially young ones, who agree in big numbers that Trump is a racist and a sexist, unqualified for America’s highest office – but who are refusing to back Clinton. One survey of 18- to 34-year-olds recorded dire numbers for Trump, but still found only 31% supporting Clinton, with 29% preferring libertarian Gary Johnson and 15% the Green candidate Jill Stein. If those numbers are replicated on election day, Trump will win.

This too stirs queasy memories of 2000. In that year, a small but significant bloc on the left voted for Ralph Nader instead of Gore. More than 90,000 voted for Nader in Florida, a state Gore was eventually deemed to have lost by just 537 votes. Had even a tiny fraction of those Naderites decided to hold their nose and choose Gore instead, there would have been no Bush presidency – and no Iraq war.

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I know this is a sore, and contested, point among those who chose Nader, who to this day blame Gore for not doing enough to appeal to the left (forgetting perhaps that he also had to appeal to centrist voters in order to win the election). But the basic case is surely clear. In a two-party system like America’s, if you want to stop Trump, then a vote for Johnson or Stein will not do it. Only a vote for Clinton can prevent a white nationalist bigot becoming the next US president.

You might think this is all so obvious that it barely needs stating. But 2000 can seem like ancient history to those who don’t remember it. And here, younger voters are not to blame. Older liberals haven’t seared the Bush calamity into the minds of the next generation. They never turned 2000 into a cautionary tale of the cost of third-party indulgence. As the New Republic put it this week, “Liberals have failed to teach millennials about the horror of George W Bush.” (Republicans, meanwhile, bang on about the supposedly dismal presidency of Jimmy Carter to this day, just as British Conservatives invoked the 1979 “winter of discontent” for over three decades. It is the centre-left that is oddly reticent about exploiting its opponents’ history of failure.)

So now, even though the hour is late, this is the case Democrats must make. That this is an exceptional election because Donald Trump represents an exceptional menace, to America and to the world. It is indeed a referendum on Trump and the only way to vote no in that referendum is to vote Clinton. She may not be the candidate of your dreams, but she’s all that stands between you and an American nightmare.

You don’t have to love her. You don’t even have to like her. You’re not saying she’s flawless (though the current Democratic platform is its most progressive in 30 years, thanks in part to the challenge of Bernie Sanders). As Joe Biden puts it, “Don’t compare her to the Almighty. Compare her to the alternative.” And the alternative is President Trump.