Before he joined the White House as Donald Trump’s chief economic adviser, Gary Cohn hit a ceiling on Wall Street. After a rapid ascension from selling aluminum siding and window frames to dealing options at the New York Mercantile Exchange to the No. 2 spot at Goldman Sachs, Cohn stalled: even if longtime C.E.O. Lloyd Blankfein had decided to step down, which he never did, Cohn’s “brusque manner”—he was known as a blunt operator who would often “plant his foot on a trader’s desk, his thigh close to the employee’s face, and ask how markets were doing”—apparently didn’t endear him to the board. So when Trump, somewhat unexpectedly, came calling, Cohn quit Goldman for another path to power.

That gamble, perhaps predictably, backfired: Initially celebrated as the West Wing’s resident “adult in the room,” Cohn’s credibility was relentlessly eroded in a series of humiliating episodes. He abandoned his principles to defend Trump’s decision to withdraw from the Paris climate accord (“We want to be in the coal business,” the lifelong Democrat found himself telling reporters, days after he’d said that “coal doesn’t even make that much sense anymore”). He credited the president for stock market gains that had, as he well knew, been ongoing for years. In August, after agonizing over the president’s equivocal remarks about white supremacy, Cohn ultimately chose not to quit. Trump rewarded his loyalty by giving the Federal Reserve chair job to Jerome Powell.

Now, all that is left for Cohn is selling the president’s increasingly unpopular tax cut. But Cohn, who is many hundreds of millions of dollars richer than he was in his aluminum siding days, seems to be struggling to connect with the middle class workers that Republicans insist they are trying to help. In April, when Team Trump first unveiled its one-page, double-spaced, bullet-point outline for tax reform, Cohn staunchly defended the repeal of the estate tax, for instance, as a boon to struggling farmers. But in an interview Thursday morning with CNBC’s John Harwood, Cohn’s arguments broke down under the slightest prodding, revealing his transformation from straight-shooting pragmatist to Trump’s new Baghdad Bob: