At the end of “The Candidate,” the engrossing 1972 film about the underbelly of campaigns, rookie politician Robert Redford is shocked by his triumph. Grabbing his handler, he pleads, “What do we do now?”

In a case of life imitating art, Donald Trump finds himself in a similar situation. His improbable run is rocking and shocking the world, but after each victory, the climb only gets steeper.

The home stretch of the nomination race is going to be ferocious, and if he survives, he faces a general-election war against Hillary Clinton that would be unlike any in modern times.

Is Trump built to go the distance? Can the author of “The Art of the Deal” close the deal of a lifetime?

Yes he can. He can get a majority of delegates and beat Clinton, too. But he’ll have to tone down the juvenile nastiness, flesh out and stick to clear policies and build a national campaign infrastructure. Oh, and he’ll need to spend real money, at least $1 billion, either his own or other people’s.

The list is a tall order, but doable. Moreover, Trump starts this new phase with advantages, not the least of which is a passionately devoted base of support and the fact that he won’t need to make the usual tricky pivot toward the center for the general election.

Despite a hard line on immigration, he’s already close to the center in other ways, with his support for the non-abortion work of Planned Parenthood a prime example. On other conservative litmus tests, including taxes and gun control, his is the most liberal record a Republican front-runner has had in more than half a century, so supporting him in the general wouldn’t be an impossible leap for moderates.

That head start would give him an edge over Clinton, who faces a lot of repair work after she finally dispatches Bernie Sanders. Her drift toward socialism to head him off leaves her far from the center, where most independents are.

But it won’t be easy for her to move right because the Sanders wing, which doesn’t trust or like her, won’t give her a free pass. Those hurdles don’t include the potentially fatal FBI probe hanging over her.

But Clinton is not Trump’s immediate problem — consolidating the fractured Republican Party is. Reports of the GOP’s imminent demise are frequent, but not yet irreversible fact. There is still time for Trump to demonstrate that he is a leader and not just a human wrecking ball.

Roughly speaking, he must fix three problems:

First: His me-me-me ego can be a turn-off to Main Street Republicans who are culturally wary of braggarts, so he needs to talk less about himself and more about the Americans he’s trying to enlist. Off-the-cuff is interesting until he wanders off into the weeds of TrumpLand.

Second: His stumbles and lack of knowledge on foreign affairs give cold shivers to policy and intellectual conservatives. He needs to develop a brain trust that can get him up to speed on major hot spots. Respected advisers can also act as surrogates to comfort others who don’t know or trust a candidate.

And, third: His relationship with congressional Republicans, which is somewhere between frosty and nonexistent. The aim of holding the Senate should be a common ­denominator, especially with the Supreme Court in the balance. As virtual head of his party, Trump would need to keep congressional candidates’ interests in mind.

Now is the time to address those concerns and stop making new enemies. Already he faces upwards of $20 million in negative ads in Florida that aim to hand the winner-take-all delegate pot to Marco Rubio. And there’s talk that Rubio and Ted Cruz will take a dive in Ohio so Gov. John ­Kasich doesn’t have to split the anti-Trump vote there.

The aim is to block him from getting a majority of delegates and set up a contested convention. The weaknesses of the plan are many, including that there is no broadly acceptable alternative to Trump. Mitt Romney would be run out of town by Trump’s voters, and there would be a lasting schism, and a Clinton victory, if a maneuver hands the nomination to a rival who has fewer delegates than he does.

Those factors make consolidation difficult, but necessary. One poll found that 27 percent of GOP voters would pick Clinton over Trump, a shockingly large defection he must prevent.

Luckily for him, he can kill two birds with one stone. In addition to helping him secure the nomination, beginning the hard work of uniting the party would bolster his chances in the fall.

Attracting independents and crossover Democrats is important, but there aren’t enough available, so he needs to unite the GOP and build on it.

By all indications, he realizes as much. At Thursday’s bruising debate, he stressed that he’s a negotiator and that not all his positions are carved in stone, including on immigration, which had to please some who fear he is too dogmatic.

Two days before that, on Super Tuesday night, he held a press conference instead of a victory rally, and threw a change-up instead of a fastball. He was more gracious than usual and talked of uniting the party, leading a very skeptical friend to note that it was the first time he saw Trump looking presidential.

“Look, I’m a unifier,” the candidate said. “I know people are going to find that a little bit hard to believe, but ­believe me, I am a unifier.”

He added: “And we are going to be a much bigger party . . . That hasn’t happened to the Republican Party in many, many decades. So I think we’re going to be more inclusive. I think we’re going to be more unified.”

The numbers make his point. On Super Tuesday, Republican turnout increased by 81 percent, from 4.7 million in 2012 to 8.5 million last week, according to the Wall Street Journal. Trump is clearly the straw stirring the GOP drink for good.

And for bad. In late January, before voting started, Gallup found that he had a 60 percent unfavorable rating among all voters, the highest it has found among any major candidate going back to 1992. By contrast, Hillary Clinton’s highest negative has been 53 percent, and in 2012, President Obama and Romney both maxed out at 48 percent.

Of course, Trump must broaden his base without losing too much of the street-fighter attitude that is key to his appeal. He will have to hold his tongue and control his hyper-competitiveness by reminding himself that the election is about America.

To keep his eye on the prize, he might channel his inner Vince Lombardi, the legendary football coach who also was excellent at closing the deal. “Winning isn’t everything,” Lombardi said, “it’s the only thing.”