Donald Trump has been known for his controversial comments that make headlines — but his positions on some key issues, like abortion and health care, appear radically different through the years. Here's a look at some of Trump's takes on the issues, then and now. (Julie Percha/The Washington Post)

Donald Trump wants to make America great again. This is how he wants to do it:

If Trump were elected president, he says, he would launch the U.S. government into a massive building project — and a massive manhunt — both at once.

On the U.S.-Mexico border, Trump would build a long, impenetrable wall. In the rest of the country, he would pressure the estimated 11 million illegal immigrants to “self-deport” — and, if they don't, round them up to deport en masse. Later, Trump says, “the good ones” could come back.

He also wants to go on a building spree.

Modern new VA hospitals. Better bridges, highways, railroads. A new floor at LaGuardia Airport, to replace that shabby terrazzo Trump hates. And, to pay for it all, Trump would not raise taxes. He’d lower them.

Instead, Trump would get other countries to start paying the United States large new sums of money — and agree to receive nothing in return. China, for instance, would pay for new tariffs. Mexico would even pay for America’s new border wall.

“They’re not going to pay for the wall,” Fox News host Bill O’Reilly told Trump this summer.

“You have to let me handle that, okay?” Trump said.

Trump, a billionaire real-estate developer and ­reality-TV star, has surged to the lead in the Republican presidential nominating contest using a showman’s flair and anti-immigrant and anti-Washington rhetoric.

But, so far, he’s missing something basic: a policy platform. A formal list of Trump’s ideas for America.

“They’re all done,” Corey Lewandowski, his campaign manager, said in a recent radio interview. Not that they’re going to share them or anything. “They’re done and we’re waiting, you know, for our schedule,” Lewandowski said.

Nonetheless, Trump has already explained pieces of his vision — in this year’s speeches and interviews, and in “Time to Get Tough,” a political book he wrote four years ago.

At times, those ideas make Trump sound like a conservative Republican. He wants to repeal “Obamacare.” He has called global warming “bull----.” He wants to end the Common Core education program and renegotiate the nuclear deal with Iran. But, at other times, he sounds more like a Democrat: Trump, for instance, rejects GOP plans to overhaul Medicare.

In other areas, Trump’s ideas seem to defy both parties’ orthodoxies. And sometimes, to defy logic.

To square the circles in his vision — to explain how he will do things that seem implausible — Trump’s answer is usually himself. That is the heart of Trump-ism, the glue that holds the agenda together — the man’s own sky-high self-confidence.

President Trump’s vision would work because Trump would be president.

“Trump is like your Uncle George at Thanksgiving dinner, saying he knows how to solve all the problems. It’s not that he’s always wrong. It’s just that he’s an auto mechanic, not a policy guy,” said Mark Krikorian of the Center for Immigration Studies, which calls for reduced immigration.

Krikorian actually agrees with Trump that illegal immigration is a major problem. But he’s frustrated by Trump’s ideas to fix it — such as a suggestion to charge Mexico $100,000 for “every person they send over” across the U.S. border.

“Charging them, how? For what?” Krikorian said. “It’s just Uncle George at Thanksgiving, kind of holding forth.”

In a phone call Saturday evening, Trump rejected arguments that his ideas — particularly those about building bigger projects on lower taxes — were implausible.

“It’s not implausible, because we’re going to make the economy sing,” he said. Trump said that under his leadership, more jobs and more growth would make up for lower tax rates.

He was asked whether — in the course of his campaign — he had encountered any issue where the situation was more complicated than he’d first believed. The answer seemed to be no.

“Life is complicated. But this is not complicated, believe me,” he said. He assured a reporter that he would like America under Trump: “You’ll be happy, believe me. You’ll be happy.”

The candidate was in Scotland late last week, visiting one of his golf courses. In his phone call with The Washington Post on Saturday, Trump was asked about a question raised by conservatives.

If he became president, were there any major parts of the federal government he wanted to cut?

Trump said he would cut portions of the Education Department — where he wants to eliminate Common Core — and parts of the Environmental Protection Agency. He cited clean-air enforcement as an area where regulators have become too burdensome.

For now, Trump’s lack of a detailed campaign platform hasn’t stopped his campaign. He leads the Republican field by at least six percentage points in poll averages. That would be enough to put him at center stage for the first presidential debate on Thursday night.

“It’s not so much what I like about him. It’s what I dislike about everything else in Washington,” explained Skip Houston, 51, an airline pilot from Georgia and a Trump supporter. Houston said he admired Trump’s raw approach and his ridicule of Washington’s culture of fundraising and favors.

But what about Trump’s policy ideas? “He hasn’t really gotten that in-depth yet,” Houston said.

Did that bother him? “It’s too early” Houston said. “There’s over a year to go.”

One thing is clear: Trump has reversed several positions from his past.

In 1999, contemplating a possible presidential run, Trump said he was pro-choice. Today, he is against abortion. He previously praised the idea of a national, single-payer health-care system. Today, while aiming his fire at the president’s health-care law, he doesn’t.

Trump also seems to have backed off another unusual idea: a one-time mega-tax on the nation’s very wealthy. In 2000, Trump advocated a 14.25 percent tax on people with a net worth over $10 million, which he estimated would raise $5.7 trillion and pay off the national debt in one swoop.

“It’s a win-win for the American people but an idea that no conventional politician would have the guts to put forward,” Trump wrote in his 2000 book, “The America We Deserve.”

Trump doesn’t mention that idea now.

“The numbers no longer work,” because the national debt is higher, said one longtime friend, who asked for anonymity in order to speak about Trump’s thinking without authorization from his campaign. “It was a good idea at the time. We should’ve done it. We’d be out of debt now. Or maybe not.”

Today, when Trump talks about taxes, he usually talks about lowering them.

He has called for eliminating the estate tax. Lowering personal income taxes. Other Republicans say the same. But Trump has gone much further in one respect. In his 2011 policy book, “Time to Get Tough,” Trump called for eliminating the income tax on corporations entirely: “A zero percent corporate tax would create an unprecedented jobs boom,” he wrote.

But it would also open a sinkhole in the federal budget. About 9 percent of all government revenue would vanish.

At the same time, Trump has contemplated expensive new plans. To fight the Islamic State, for instance, he has advocated a military campaign aimed at removing the oil out from under the militants’ territory.

“Take back their wealth. Take back the oil. . . . You bomb the hell out of them and then you encircle it, and then you go in” with an oil company, Trump has said. “Once you take that oil, they have nothing left. And it’s so simple.”

Oil-industry experts expressed skepticism about this plan. Skepticism, in fact, may not be a strong-enough word. They noted, for instance, the difficulty of finding a company willing to get oil out of an active war zone, and that depleting the area’s relatively minor oil fields might still take decades.

“That is sheer lunacy on so many counts, it’s hard to start,” said David Goldwyn , a former State Department special envoy for energy in the Obama administration.

At the same time, Trump — a real-estate developer now imagining himself in charge of the largest property owner in the country — has a lot of ideas for things to build at home.

Highways. A fortified border. Better VA hospitals to replace what Trump called “outdated dumps.”

“When he comes up against a problem, his reflexive answer is that we’ll do something to fix it that’s going to cost more money,” said Michael Tanner, of the libertarian Cato Institute.

He said Trump had not explained enough about how these big projects would be paid for as tax revenue declined. “You can’t spend more and collect less. That’s kind of basic math,” Tanner said. “You can argue about how the math adds up in the other people’s plans. But there’s math there. This, there’s just no math.”

That’s where the other countries come in.

In his 2011 book, for instance, Trump called for a 25 percent tariff on all goods imported from China if China wouldn’t stop unfair trade practices. Trump has also called for increased tariffs on imports from Mexico in order to pay for his wall.

He would also threaten American companies with tariffs if those companies wanted to shift U.S. jobs overseas.

“I would call up the head of Ford, who I know. If I was president, I’d say, ‘Congratulations. I understand that you’re building a nice $2.5 billion car factory in Mexico and that you’re going to take your cars and sell them to the United States, zero tax,’ ” Trump said during his campaign announcement. “So I would say, ‘Congratulations. That’s the good news. Let me give you the bad news . . . we’re going to charge you a 35 percent tax.’ ”

But these plans would not be easy. Even for President Trump.

Even if Trump could get Congress to approve new tariff increases, for instance, they would likely violate existing trade agreements. And they would hit Americans in the pocketbook by making imported goods more expensive. And, probably, they would trigger retaliations from other countries, which would raise their own tariffs and hurt U.S. exports.

“If you thought this had a ghost of a chance — which it doesn’t — you would sell all your stocks,” because of the damage that a trade war would do to the U.S. economy, said Gary Hufbauer , of the Peterson Institute for International Economics.

Hufbauer said that the United States has spent decades trying to lower tariffs worldwide. “This would be the U.S. kind of going into the insane asylum,” he said. “It would be blowing up the system which we have created since the Second World War.”

In Trump’s plan, the thing that makes it all work is . . . Trump. This confidence is not new. Back in 2000, Trump wrote that — if elected president — he would appoint a new U.S. trade representative.

Guess who.

“My lawyers have checked, and the president has this authority,” Trump wrote then. “Our trading partners would have to sit across the table from Donald Trump.”

This year, Trump said he believed those same deal-making skills could also improve U.S. relations with an unpredictable rival: Russian President Vladi­mir Putin.

“I would be willing to bet I would have a great relationship with Putin. It’s about leadership,” he told Fox’s O’Reilly.

“Based on what?” O’Reilly asked. “You’re two macho guys?”

“Based on a feel.”

“Just a feel?”

“Based on feel,” Trump replied.