One recent weekday afternoon, Lloyd Blankfein was sitting in his office in Goldman Sachs’s gleaming $2 billion headquarters off of the West Side Highway, in lower Manhattan, discussing one of the only subjects that bankers, financial executives, and most of the sentient world can talk about these days: Donald J. Trump. Blankfein, a Democrat who supported Hillary Clinton, had previously expressed discontent with the president after his botched travel ban was rolled out. But on this spring day, that fraught executive order seemed like a veritable lifetime ago. After all, since then there have been the various contretemps over Mike Flynn, the vicissitudes of Sean Spicer and Steve Bannon, and the hijinks surrounding “smart cookie” Kim Jong Un, among other episodes. Then, of course, there was Trump’s abrupt firing of F.B.I. director James Comey, who had recently asked for increased resources for the bureau’s investigation into any meddling between Trump’s campaign and Russia. (All of which occurred before Monday’s bombshell that the bad hombre in chief had spilled some state secrets to the Russians during last week’s Oval Office meeting, and Tuesday’s bombshell that Trump had asked Comey to shut down his investigation into Flynn.)

These days, reading the tea leaves of Trumpworld is a daily, if not hourly, obsession. This is doubly true on Wall Street, where the president’s tweets can shuffle billions of dollars across the markets. For many, decoding Trump is like conjugating irregular verbs in Latin. But Blankfein may have more purchase on what’s going on inside the White House than many of his colleagues. For all of Trump’s Jacksonian promises to revert to an era of economic nationalism, the president has, of course, more or less forked over the responsibility of running the country’s finances to a team of Goldman alums—Blankfein’s former right-hand man, Gary Cohn, the national economic adviser; Steve Mnuchin, now the Treasury secretary; Dina Habib Powell, a deputy national security adviser; and, of course, Steve Bannon, whose tenure at Goldman was short and unmemorable.

Sitting in his office, Blankfein explained that Trump’s unconventional behavior could easily disrupt his proposed legislative agenda, particularly the part that Wall Street cares most about: the promise of tax reform, regulatory reform, infrastructure revitalization, and other pro-business measures that have sent the Dow Jones Industrial Average soaring up nearly 14 percent since Election Day. Obviously, if Trump is stymied in his effort to pass this agenda, you can kiss good-bye the booming stock market. “The market is moving,” Blankfein said. “Everybody knows what’s being sought, what the president wants to do, and I think the market is discounting and recalibrating every minute what they think his effectiveness is going to be.” Recent events, he noted, were likely to dampen the likelihood of achieving these goals. Legislative success is contagious, he told me. “But the opposite is also true. “If he loses a vote, or if it looks like he’s less likely to win a vote or his effectiveness is waning,” he continued, then that could affect Trump’s ability to get things done.

Still, despite the ongoing turmoil that Trump causes in the White House, Blankfein said that the president might yet emerge with some victories, perhaps owing to the Darwinism of congressional Republicans who fear reprisals at the ballot box next year. Blankfein imagined aloud what the calculus might be for such G.O.P. legislators. “If I was holding out for a whole loaf, but at some point, if the clock is running out and I have no chance of getting a whole loaf, I’ll take whatever part of the loaf I can get,” he said. “I don’t want to go home and say I’ve got nothing to show for my last two years of my party in power.”

Blankfein was more optimistic about some measures than others. “I think there’ll be some tax reform,” he said. “Will it be massive tax reform? Will they get the rate as low a rate as he wants? I’m not sure.” (Trump has proposed reducing the corporate tax rate down to 15 percent).