For years, I believed Democrat Bob Kerrey, the former governor and senator from Nebraska, would succeed in his quest to be president. After it was clear that it wasn’t going to happen, I asked him why.

His answer was both self-serving and true: “You have to want it more than life itself,” Kerrey said without hesitation.

It was obviously a question he had thought about a lot, and it’s one that Donald Trump now faces. How much does he want to win? Is there a limit, in money, discipline and humility, beyond which he refuses to go?

The questions are urgent because Trump is on a path to defeat — and not just an ordinary one. He’s headed into a crushing landslide that would turn his name into a political punchline and make his brand synonymous with loser. He would join the failed nominees’ Hall of Shame — Barry Goldwater, George McGovern, Walter Mondale.

Trump now trails in all 11 states that Politico identifies as battleground states, some by double digits and some where he has fallen below 40 percent. He’s actually losing support, putting several reliable red states in play, and his electoral map is shrinking so fast that pollster Doug Schoen predicts that Hillary Clinton could get 400 electoral votes, far beyond the 270 needed.

In response to his rapidly deteriorating position, Trump brought in two new top aides and opted for more substantive policy speeches rather than rah-rah rallies. The speeches, on fighting terrorism and liberating cities from lawlessness and decay, were solid and offer a clear contrast of ideas to the status quo and to Clinton.

Yet they are exceptions to an otherwise shapeless campaign, and I don’t see any signs that Trump is totally dedicated to victory. Indeed, reports say the candidate felt “boxed in” by his former top staff and blames them for his predicament.

Strangely, or perhaps not, he even vows that he won’t change a thing, saying Tuesday, “I am who I am.”

That is either face-saving bravado — or a prelude to a historic thumping. For unless Trump changes, the worst is yet to come.

For one thing, he is still boasting about the size of the crowds he draws, but having several thousand supporters stuff an auditorium is ultimately meaningless. Consider that Mitt Romney got 61 million votes four years ago and got crushed.

For another, the Clintons are succeeding in painting Trump as a dangerous and toxic alternative. Does he not see that, or does he harbor the illusion, like Romney did, that all the polls are wrong?

Some of the changes Trump must make are elementary. He would look less static by taking off his blue suit jacket and red tie and rolling up his sleeves on his white shirt.

While he doesn’t have to kiss a baby or eat a deep-fried Twinkie, getting out from behind the podium to wade into crowds of supporters would create a more active image to match his populist rhetoric.

Trump is still compelled to trade insults with every critic, and while he’s clearly right that the media is biased against him, his incessant whining is starting to sound like an excuse for defeat.

More fundamentally, the Kerrey standard is about what parts of his mental and emotional self Trump is willing to sacrifice. It is not a demand for compromise but for commitment, a willingness to do whatever it takes.

Again, I don’t see the evidence. Trump is still compelled to trade insults with every critic, and while he’s clearly right that the media is biased against him, his incessant whining is starting to sound like an excuse for defeat.

Ditto for his talk of a “rigged” election. Is he committed to winning or preparing to explain a loss?

Then there’s the smartest-man-in-the-room syndrome. Can he take advice from people who disagree with him, or does he only trust himself, even though his own instincts led him into this current sinkhole?

None of this is meant to minimize Trump’s enormous achievement. The ultimate long shot, he blew away a crowded field of talented Republicans and brought millions of new voters into the primaries. The issues he raised — trade, immigration and law enforcement— are now on the national agenda, with or without him.

But that’s what Bernie Sanders got — platform sops and platitudes as consolation prizes. It’s nice, but it’s not winning.

Trump is the last hurdle to a Clinton presidency. He alone will bear the responsibility if her scandal machine ends up in the Oval Office. His children and grandchildren will be saddled with that unhappy legacy.

This is a very winnable race, or was until Trump decided that being himself was the most important thing of all.

More important, apparently, than delivering on the promises he made to the millions of working-class supporters he pledged to represent. More important than fixing the American economy and securing the homeland. More important than restoring the nation’s pride and, yes, more important than making America great again.

He still has a shot at all that — or he can just spend the next seven weeks indulging himself. In that case, his defeat will be the ultimate betrayal.