As recently as three weeks ago, it was looking as though none of the laws of political physics applied to the phenomenon that is Donald Trump.
But the days since his strong showing in the March 16 round of primaries have seen the GOP frontrunner make a series of stumbles over his own feet.
No longer does he appear to be invulnerable to gaffes and mistakes that would have destroyed a more conventional candidate before the Iowa caucuses.
That is why Trump's second-place finish to Texas senator Ted Cruz in yesterday's Wisconsin Republican primary may represent no ordinary setback. It appears to be a pivot point - although it has yet to be seen whether the trajectory from here points downward or upward.
"Either these are the weeks we discovered he had weaknesses he couldn't overcome, or these are the weeks when he and his team realised they had to get better," said former House Speaker Newt Gingrich.
"If he makes the transition to being a really professional presidential candidate, he will be really formidable. And if he does not, he will not be the nominee."
Trump campaign manager Corey Lewandowski insisted that there would be no major shift in strategy.
"Absolutely not. We have from day one run the same campaign," Lewandowski said. "We're going to continue to execute. We're not going to blow up the model."
Yet some recalibration is under way. Trump thus far has offered few specifics on how he would achieve his grand goals.
But yesterday he outlined for the first time how he would compel the Mexican Government to pay for the wall he proposes for the border between Mexico and the United States, saying he could do it by threatening to cut off the billions of dollars that Mexicans in the US send back to the country.
He plans to be more specific in coming weeks, with a series of detail-rich speeches on strengthening the military, reforming education and spelling out his criteria for picking justices for the Supreme Court.
Lewandowski described the new emphasis as representing "the natural maturation of the campaign". As usually happens when a campaign hits a rough patch, there have been predictions of campaign-staff shakeups in the offing.
But in Trump's case, there is only one direction in which the finger of blame can be pointed.
The strength and the weakness of his unconventional campaign has been Trump himself. He has functioned in many ways as his own chief strategist, political consultant, policy tsar and communications guru.
That is the same way he built his fortune, said campaign spokeswoman Hope Hicks.
"In real estate, he builds a building. It doesn't matter who the marble vendor is, or who the carpet vendor is, or who's providing the furniture, or who is the designer. His name is on the building, and it's his product," Hicks said.
"He sort of approached the campaign very similarly: 'My name is on this, and nobody else really matters'."
The limitations of that approach have become more apparent as the campaign has been spread more thinly and as the field of competitors has narrowed.
Trump's foes also have seized upon new signs of vulnerability, spending millions to amplify their advertising assaults and organising efforts in Wisconsin and elsewhere.
In Wisconsin, Our Principles PAC, an anti-Trump group, spent US$2 million ($2.9 million) and had a robust, across-the-board operation: television and digital ads, billboards, direct mail, phone calls, and absentee and get-out-the-vote programmes.
"Trump didn't take the time to build out an organisation that could go all the way, and he doesn't have the infrastructure in various states, to the point he's not fielding full delegate slates," said Brian Baker, a senior strategist for the group.
Baker said he saw Trump's recent hiring of Paul Manafort, a veteran GOP consultant who will be working on convention and delegate issues, as "a de facto acknowledgment that it's too late to get an organisation together and there will be an open convention".
Trump and his team also have made a stream of missteps and misjudgments of late.
He retweeted an unflattering picture of Cruz's wife, prompting the Texas senator to call Trump a "snivelling coward" and warn him to "leave Heidi the hell alone".
Trump, who rarely owns up to making a mistake, later admitted that the tweet about Heidi Cruz had been one.
Trump brought his own wife, Melania, onto the campaign trail in an apparent effort to boost his low standing with female voters.
David Winston, a veteran pollster who works with congressional GOP leaders, noted that the gender gap is emerging as a critical vulnerability for Trump.
"When we're talking about women, we're talking about the largest voting bloc in the country. The gender problem is a problem with the majority of the electorate," Winston said.
"Trump has made it about personality and has been very stark, being provocative in terms of creating tension."
Violence and chaos have became commonplace at his campaign events, including an incident in which Lewandowski was caught on video grabbing the collar of a protester.
Ten days later, Florida police charged the campaign manager with simple battery against a female reporter who had been trying to ask the candidate a question.
In both instances, Trump has come to the defence of his embattled campaign manager.
Trump has shown an uneven grasp of policy and world events in interviews with major news organisations, including the Washington Post and the New York Times.
He has also botched questions on abortion, managing to draw fire from those on both sides of the issue - arguing that women who get abortions should be punished, then that only providers should be, and then that law on the issue has been set.
"At some point, the candidate's words must stand on their own. What do you believe, Mr Trump? No one knows," said Marjorie Dannenfelser, president of the Susan B Anthony List, a leading anti-abortion group.
Meanwhile, Trump has been outmanoeuvred on the ground by the highly organised Cruz campaign, which used its superior grasp of party rules to garner more delegates in Louisiana, although Trump won the statewide vote.
And at times, he has appeared to be coasting, taking a full week away from the trail after winning the Arizona primary on March 22. Cruz and Ohio Governor John Kasich used that time to press for votes in Wisconsin.
The coming weeks will take the campaign to what should be more favourable terrain for Trump, including his home state of New York. And Trump professes not to be dismayed by the setbacks.
"I've been given the last rites how many times, like 10? Every week, it's the end of Trump," the billionaire said during a rally in Superior, Wisconsin. "They walk in, 'Sir, I don't know what happened, but your poll numbers just went through the roof'."
Making it count
1237 delegates needed to win the Republican nomination
• Donald Trump has 740 delegates
• Ted Cruz has 514
• John Kaish has 143
2383 delegates needed to win the Democratic nomination
• Hillary Clinton has 1743
• Bernie Sanders has 1056