By no conceivable measure is President Donald Trump a smart person. But he does have certain gifts, one of which we're seeing in operation right now. It served him well as a businessman and affects almost every aspect of his presidency: Trump has an almost unfailing internal homing device tuned to the vulnerabilities in people and systems.

That's also what's driving the Ukraine scandal - and what may help him escape it.

It comes from a lifetime lived beyond the reaches of rules, norms, accountability and even the law. It comes from Trump's worldview, in which every interaction between individuals, groups or institutions is zero-sum. He's going to win and you're going to lose, and in order to make that happen, he is constantly on the lookout for vulnerabilities he can exploit.

In Ukraine, Trump found a country that was deeply vulnerable and therefore could be enlisted to help his reelection campaign. Sitting next to Russia, an adversary both highly aggressive and far more powerful, Ukraine needs help from the United States. From the beginning of the Trump administration, the Ukrainians seemed to understand that getting that help required a submission to Trump's personal interests.

Last May, we learned that Ukraine ceased its cooperation with the special-counsel investigation and suspended its own investigation into former Trump campaign chairman Paul Manafort, who had helped since-deposed Ukrainian president Viktor Yanukovych loot the country before Yanukovych fled to Russia.

Why did they do it? "In every possible way, we will avoid irritating the top American officials," said one Ukainian official. "We shouldn't spoil relations with the administration."

At that time, Ukraine was negotiating the purchase of anti-tank weapons from the United States, which could provide deterrence to a Russian incursion. But the Ukrainians' need for U.S. support was ongoing, as Trump well knew when he dispatched Rudy Giuliani to pressure them to dig up dirt on Joe Biden's son.

Both Giuliani and Trump have admitted that that's exactly what they were doing. What's more, it was earlier reported that "The current prosecutor general [of Ukraine] later told associates that, during one of the meetings, Mr. Giuliani called Mr. Trump excitedly to brief him on his findings." This was a project in which the president was intimately involved.

In other words, at some point Trump realized that Ukraine's position made it vulnerable; needing U.S. help, it could be enlisted to aid his reelection campaign. But that's just the beginning.

As Trump understands as well as anyone, the news media is vulnerable to his manipulation. He can make wild, unfounded accusations about Biden or anyone else, and those accusations will be repeated and spread, with factual corrections left for the 12th paragraph or a fact-check most people won't read. It's already becoming clear that much of the news media is going to impose an utterly preposterous false equivalence on this story that will encourage people to think it's just one more Washington squabble where both sides are on equal moral footing.

Trump also sees the vulnerability of Republicans in Congress. Their constituents have been well instructed that there is no catastrophe worse than Democrats winning an election and no crime that would not be justified in order to prevent that outcome. When you watch some pathetic Republican officeholder struggling to explain away Trump's misdeeds, it's not only because he lacks moral courage but also because he knows opposing Trump would be political suicide for him. He has no choice but to acquiesce, and Trump knows it.

And Trump understands the vulnerability of the entire system. Not only can norms be broken without consequence, but so can the law itself. Trump can simply refuse to comply with congressional subpoenas and dare Democrats to sue. They might win in court (unless they run up against a compliant Trump-appointed judge), but so what? The process will take months or even years to play out.

And if Trump's director of national intelligence just decides that he'd rather not comply with the law mandating that the whistleblower complaint on Trump's phone call with the Ukrainian president be turned over to Congress? Then he won't.

Trump has come to understand that the system is vulnerable to a president who believes that the law doesn't apply to him, because many of the mechanisms to restrain him are almost toothless if he decides not to obey. And why wouldn't he believe that? He got elected with the help of a hostile foreign power, he uses the presidency to enrich himself, he hides his tax returns from public view, and when his opponents say "You can't do that!," he says, "Yeah? Watch me."

So often in Trump's business career, we saw how he identified others' vulnerability and exploited it. He would contract with small-business owners, then once they had provided their goods and services, he'd simply refuse to pay them. He did this hundreds of times, knowing that the business owners were vulnerable because they couldn't afford to pay for the lawsuits it would take to force him to pay up.

He knew that the suckers he roped in to scams such as Trump University were vulnerable, made so by their admiration of his celebrity and their own economic desperation. He knew that the undocumented workers he employed were vulnerable because of their immigration status. And of course, he knew how vulnerable women are to being sexually assaulted.

A lifetime of breaking rules and laws, and getting away with all of it, has taught Trump that he need not ask "Should I do this?" but instead should ask only "How can I get away with this?" Again and again, the answer has been to find others' vulnerability and exploit it. It's happening again, right in front of our eyes. And it'll keep happening until we decide to stop it.

Paul Waldman is an opinion writer for the Plum Line blog.