With its budget slashed, the I.R.S. has pulled back across the board — except for one area where it’s been easier to keep the numbers from falling so much: audits of the poor. More than one-third of all audit targets claim the earned-income tax credit, one of the nation’s largest anti-poverty programs. By the hundreds of thousands, I.R.S. computers spit out letters that require low-income taxpayers to prove their eligibility. The counties with the highest audit rates aren’t found in the hedge fund precincts of Connecticut or the lobbyist enclaves of Northern Virginia. No, they’re rural, mostly African-American counties in the Deep South.

There’s nothing easy about auditing the rich. Even when the I.R.S. has evidence of flat-out cheating, such as when it discovers a hidden Swiss bank account, it can take years of work to prove its case. Taking that kind of time clashes with one of the core ways the I.R.S. evaluates its managers and agents: how efficiently they open and close audits. As one senior manager put it to us, “If you’re not churning out the exams, you have to explain why you’re not.”

But more often, the affairs of the superwealthy are infernally complicated and the I.R.S. doesn’t have black-and-white proof of evasion. Instead, the agency runs up against what’s called “aggressive” planning in the tax world. To the rich, tax returns are just opening offers to the government. If they don’t get audited, that’s great. If they do, well, they’re ready for a fight.

In 2009, the I.R.S. decided to take a bold, new approach to auditing the superwealthy. The agency’s idea was to bring specialists together and form a kind of green-eyeshade-wearing Delta Force. It would finally do something it — surprisingly — hadn’t been doing before: look at the entirety of a taxpayer’s empire. That meant examining how his partnerships, limited liability companies, foundations, gifts and overseas operations interact with one another. The ultrarich weave all of these together so that they end up with an ultralow effective tax rate. “We hadn’t really been looking at it all together, and shame on us,” Steven Miller, a former acting commissioner of the I.R.S., told us.

Because of budget cuts and lobbying, this elite force never came together. The agency hoped to have a staff of about 240 expert auditors by 2012. But by 2014 it had only 96, and by last year the number had fallen to 58. Initially spooked by the group’s ambition, tax professionals who counsel the wealthy now speak of the effort with a kind of pity.

Criminal tax enforcement has also faltered. More than 150 million income tax returns were filed in 2017, but the I.R.S. brought fewer than 800 cases in which someone was charged with making legal income but criminally evading taxes. That’s one reason tax cheats can get away with brazen behavior for so long — until they attract the attention of federal investigators for other reasons. That’s what happened to the former Trump campaign chairman Paul Manafort and the former Trump lawyer Michael Cohen, who pleaded guilty to tax evasion, and the celebrity lawyer Michael Avenatti, who was charged with failing to file taxes at all for several years. (Mr. Avenatti denies the charges.)

So, what to do? One 2020 candidate already has a bold proposal to resuscitate the I.R.S. It’s a plan to pump tens of billions into the agency, enough to fund a second army of agents. That candidate’s name is Donald Trump.