On July 10th of last year, six days before Donald Trump confirmed rumors that he was entering the 2016 Presidential race, David Cay Johnston, a Pulitzer Prize–winning investigative reporter, published a piece at The National Memo that was headlined “21 Questions For Donald Trump.”

Johnston’s article was based on almost three decades of following Trump’s business career and writing about him. The topics he raised ranged from a bribery investigation in New Jersey during the nineteen-seventies, to allegations that a Mafia-related company helped to build Trump Tower, to the contents of Trump’s tax returns, to his failure to contribute a single dollar to his eponymous charitable foundation since 2006. “Reporters, competing Republican candidates, and voters would learn a lot about Trump if they asked for complete answers to these 21 questions,” Johnston wrote.

For more than eight months, amazingly enough, few of Trump’s rivals in the G.O.P. race, or the news organizations that were covering his campaign, bothered to seek answers to Johnston’s questions, which, in fact, cover only a subset of the pertinent issues raised by Trump’s long career. Apart from a few jibes in the early debates about some of his casinos seeking bankruptcy protection, the Republican front-runner had been getting off pretty much scot-free.

Finally, things are changing. At Thursday night’s Republican debate in Houston, Senator Marco Rubio repeatedly jabbed at Trump’s record. He accused Trump of hiring “people from other countries to take jobs that Americans could have filled,” including some illegal immigrants. Rubio pointed out that some of the clothes Trump sells are made in Mexico, which he routinely accuses of stealing American jobs. He pointed out that some former students at Trump University are suing the school. And Rubio wasn’t the only one to go after Trump. After the New York billionaire cited an I.R.S. audit as the reason why he is delaying the release of his tax returns, Ted Cruz chimed in, saying, “If he has said something that was false and that an audit is going to find was fraudulent, the voters need to know.”

Summoning his trademark bluster, Trump sought to dismiss the attacks. (He called Rubio a “choke artist” and Cruz a “liar.”) But the questions will not go away. If Trump wins the Republican nomination, those questions will dog him throughout the general-election campaign. Although he likes to portray himself as a successful entrepreneur who created a vastly profitable business, many people in the business world have long regarded him as a self-promoting huckster who emblazons his name on properties that don’t belong to him and habitually overstates his net worth. Appealing to the skeptics, Rubio said that if Trump hadn’t inherited two hundred million dollars from his father, he would be “selling watches in Manhattan.”

The immediate question is why Trump is delaying the release of his tax returns. As Mitt Romney pointed out on Twitter, even if Trump is being audited, he could put out the returns for earlier years. Why hasn’t he done so? One theory is that he uses various tax dodges to pay a very low rate of tax, and he doesn’t want that known. (In the nineteen-seventies, Johnston pointed out, Trump didn’t pay any federal tax at all for several years.) Another suggestion is that it will emerge that he has contributed very little to charity, despite the fact that he has described himself as an “ardent philanthropist.”

My favorite theory is that the returns will show that Trump’s income isn’t very high by the standards of the mega-rich, casting doubt on his claim to be worth at least ten billion dollars. Indeed, he already appears to be hinting at this. During the debate, he claimed that tax returns don’t give any indication of a person’s wealth, which is nonsense. If Trump’s various businesses are worth as much as he claims they are, they must generate a great deal of revenue and income, at least some of which would be reflected on his tax returns.

Given that illegal immigration is Trump’s trademark policy issue, his employment of overseas workers also bears more scrutiny. During the debate, Rubio brought up the fact that, back when Trump Tower was being built, undocumented Polish immigrants worked on the site, some of whom were poorly treated, and he had to pay a settlement of a million dollars. That was thirty-eight years ago, Trump replied, which is true. But Trump is still engaged in three businesses—hotels, casinos, and golf courses—that employ a lot of low-paid, casual, and seasonal workers. How many of them, if any, are undocumented? We don’t know yet. But on Thursday, the Times revealed that Mar-a-Lago resort, in Palm Beach, Florida, employed hundreds of overseas guest workers—legal ones—while turning down applications from U.S. residents. This might well be the tip of the iceberg. If it is, Trump will be revealed as a blatant hypocrite.

Another aspect of Trump’s career that will be worth investigating is his dealings with politicians. At times, Trump himself has come close to suggesting that he contributed to pols from both parties because he knew they could perform favors for him. What sorts of favors? Back in the nineteen-seventies, Johnston points out, New York City Mayor Abe Beame gave Trump a four-hundred-million-dollar tax abatement to facilitate his first big real-estate deal, the conversion of a hotel next to Grand Central Terminal. During the nineteen-eighties, when the city refused to give Trump seven hundred million dollars in tax breaks for his controversial Riverside South development, he had a bitter dispute with Mayor Ed Koch. What deals were done to grease the wheels of his various developments in Atlantic City and Las Vegas, at least one of which, Johnston writes, involved a banker for a mob boss?

In the coming weeks and months, we can be assured that more journalists and opposition researchers will finally be delving into these questions, and others. For far too long, Trump was regarded as a novelty act, and he was spared the close inspection that most serious candidates for the Presidency receive. With his run of victories in the early G.O.P. primaries, the level of scrutiny is inevitably rising. We will see if he can withstand it.