Rettig, a tax lawyer with decades of experience defending wealthy clients against the IRS, has been publicly noncommittal so far. Pressed by Democratic senators at his confirmation hearing, all he would say was that “one of [his] top priorities would be to analyze the budget.” This was a stark contrast to Koskinen’s outspoken advocacy.

In the meantime, the IRS continues to shrink. Annual revenue from audits is down by about $10 billion, adjusted for inflation, since 2010, and billions more have been lost by not pursuing nonfilers and other sources of unpaid tax debts. If the IRS had maintained a level of enforcement similar to that of the years from 2004 to 2010, it would have collected about $18 billion more than it did last year, ProPublica estimates. The total shortfall since 2011 has been about $95 billion.

The true cost is likely much larger, since IRS enforcement has a magnifying effect. People who undergo audits are less likely to evade taxes in the future, just as nonfilers who are caught are more likely to file voluntarily, studies have shown. Take away enforcement, and evaders are emboldened and grow in number.

One factor that has helped obscure this deterioration is the growth of the U.S. economy, which has pushed up tax receipts since the Great Recession. The IRS took in $3 trillion in 2017, up from $2 trillion in 2011. Republicans have pointed to this as proof that nothing is amiss: “You could argue,” Representative Crenshaw said to Koskinen in a 2016 hearing, “if you collect more revenue with less money, then maybe if you had even less money you would collect even more revenue.”

But the increase in receipts is misleading. During that period, for example, the top marginal tax rate went up, so the richest taxpayers were paying more. More important, in 2011, Americans had deep losses from the 2008 financial crisis that were still depressing tax obligations. In the following years, receipts outpaced economic growth, a typical phenomenon during recoveries. Still, that increase was weaker than government analysts expected. Even before last year’s tax cuts, tax receipts as a percentage of GDP never reached the levels of the late 1990s or mid-2000s.

It will be years before we know whether tax cheating has in fact increased. The last IRS report to assess what it calls the “tax gap,” issued in 2016, analyzed the period from 2008 to 2010. It found that taxpayers had paid about 82 percent of the taxes they truly owed. If the rate of compliance in 2017 was the same, that would translate to $667 billion in missing taxes.

Even the tiniest drop in compliance would cost billions more. But no one we spoke with who has worked at the IRS thinks the drop is likely to have been small. “One day it will be clear,” Koskinen said, “but by that time, you’re in deep yogurt.”

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