“Inflation would’ve been well above their mandate, 2.5 percent and rising, at this point,” Mr. Tedeschi said. Price gains are like an aircraft carrier — they’re hard to turn around once they get going — so that would have necessitated a sharp increase in rates. Such an abrupt change could have plunged the economy into recession.

“It would certainly be painful,” Mr. Tedeschi said.

But even in that version of the world, one in which the Fed was willing to play with fire by leaving its policy totally untouched at near-zero for more than a decade, the economy could have achieved that 4 percent growth figure only absent a trade war — and even that is a stretch.

While it’s hard to gauge precisely how much Mr. Trump’s tariffs reduced growth, estimates suggest they could have shaved between 0.5 and 1 percentage point away in 2019, Mr. Tedeschi said.

All of these projections are highly uncertain — it is difficult to know how the world would have shaped up after the fact, and it is impossible to know how policies would have interacted.

And even if the basic figures are right, this scenario is unrealistic. Leaving interest rates at rock bottom would have been expected to generate unsustainable economic conditions. That runs contrary to the Fed’s very mission, given to it by Congress.

Moderate Version

In another version of the world, the Fed could have raised interest rates between 2015 and 2018, but then lowered them much more quickly in 2019 as inflation pressures remained muted. Had they dropped the federal funds rate to zero at the very start of the year, Mr. Tedeschi said, it might have added about 0.35 percentage point to growth, getting the economy up to the 2.5 percent range.

That is also far-fetched — the Fed has never slashed rates to zero outside of a recession. Doing so at a time when the economy was growing and Mr. Trump was pushing for a move would have looked overtly political, threatening the central bank’s prized independence. It could have raised the risk of higher inflation. And even if conventional models are totally broken and price pressures no longer respond to loose Fed policy, rock-bottom rates at the height of an expansion could have helped to fuel financial excesses.