The rapid rise of Mr. Muzinich, a lanky father of two from Manhattan, is emblematic of the new power dynamics in Mr. Trump’s Washington, where business experience and New York roots are often pathways to top posts. He fits the mold of Mr. Trump’s top economic advisers, Mr. Mnuchin and Gary D. Cohn, the director of the National Economic Council, both of whom made a career on Wall Street. While not a Goldman Sachs alumnus, as they are, he brings an extensive background from the world of finance, having been a banker at Morgan Stanley and the president of Muzinich & Co., an international investment firm founded by his father.
Yet for all the intricacies of Wall Street finance, Mr. Muzinich has had to scale a steep learning curve in understanding the peculiarities of the tax code and the legislative land mines that have roiled previous attempts to rewrite tax law. Some lobbyists have expressed frustration that Mr. Muzinich and his Treasury team have been overly secretive about the tax plan and that, despite his business background, he does not always appear enthusiastic to be haggling over tax code deductions with representatives of American industries.
A congressional tax staff member who spoke on the condition of anonymity to discuss private meetings said that it had sometimes been evident that Mr. Muzinich was learning on the job in a world of tax and accounting technicians, but added that he had a strong sense of business economics, asked smart questions and was comfortable with data.
His Wall Street brashness sometimes shines through when he gets into deal-making mode. Like other political newcomers in the Trump administration, Mr. Muzinich has occasionally shown frustration with the grinding pace of legislating.
On Capitol Hill, Mr. Muzinich is often paired with Mr. Cohn’s top deputy, Shahira Knight, to convey the administration’s views in big tax meetings. He speaks almost daily with Mr. Mnuchin and, according to congressional aides, it is apparent that he has the authority to speak on behalf of the secretary.
“Justin’s leadership and expertise has been essential in advancing tax reform, which has been our administration’s top domestic priority,” Mr. Mnuchin said in a statement.
Mr. Muzinich, who holds an M.B.A. from Harvard and a law degree from Yale, was relatively unknown in tax policy circles before he was plucked from Wall Street to join the Treasury Department. His most notable prior experience in the world of policy came while working on the campaign of one of Mr. Trump’s arch rivals: Jeb Bush.
Mr. Bush first learned of Mr. Muzinich because of his work as an informal adviser to the 2012 presidential campaign of Mitt Romney, another nemesis of Mr. Trump. While Mr. Trump’s personality-driven campaign was lean on policy, Mr. Muzinich was busy overseeing a team that cranked out detailed proposals for Mr. Bush on taxes, education and health care. Those attracted little public attention for Mr. Bush’s failed White House bid, but the experience gave Mr. Muzinich a taste of doing thankless work in a political caldron and managing big personalities under pressure.
One of the earliest iterations of the tax framework unveiled by Mr. Trump and congressional Republicans last month was the one that Mr. Muzinich drafted for Mr. Bush in 2015. Like the current plan, that one would have collapsed the personal income tax brackets from seven to three and lowered the rates. It would have doubled the standard deduction, reduced the corporate tax rate to 20 percent from 35 percent, and switched the United States to a “territorial” tax system, as the current plan would also do.
“What impressed me beyond his business experience, which is pretty unique, is that he’s just a policy wonk,” Mr. Bush said in an interview. “Justin has a desire to serve and he’s getting to do it at a high level, and I’m really proud of him for doing it.”
Policy has long been a passion for Mr. Muzinich, who regularly sought out opportunities while on Wall Street to publish essays on subjects ranging from foreign aid to the mandate of the Federal Reserve. Some of his earliest writing on tax policy could be considered heretical in an administration with a guiding principle of “America First.” In a 2007 Op-Ed article in The New York Times, Mr. Muzinich and a co-author, Eric Werker, called on Congress to offer tax credits to companies that build factories in developing countries and to offset the lost revenue with reductions in foreign aid.
Gregory Mankiw, the Harvard economist, called the idea clever at the time but suggested that such a concept would be panned as the “outsource American jobs to third-world sweatshops tax credit.”
Mr. Muzinich has also written extensively on issues that extend far beyond taxes. In 2010, he wrote a long essay for the Hoover Institution on modernizing nuclear nonproliferation strategy in international waters.
He grew up and studied in Democratic havens like New York City and Harvard, but Mr. Muzinich’s conservative leanings were obvious at an early age. Former colleagues describe his views on tax policy as center-right, with a focus on spurring economic growth by broadening the base of taxpayers and lowering tax rates.
Like many officials who have joined the Trump administration, Mr. Muzinich is straddling two worlds. His wife, a doctor, and young children still live in New York, and he spends his weeks living out of hotels in Washington and commuting home to the Upper East Side on the weekends. A squash enthusiast, Mr. Muzinich has little time for the courts amid the crush of meetings and number crunching, according to friends who keep in regular touch with him.
While many members of the Trump administration have become household names defending the president and his policies on television, Mr. Muzinich, who declined to be interviewed, has kept his distance from public political combat and maintained a low profile.
“I think that he is pragmatic,” said Glenn Hubbard, the dean of Columbia Business School, where Mr. Muzinich taught before joining the Treasury Department. “He’s looking for good policy solutions, not policy positions.”
Critics have accused the Treasury Department of taking a different approach. Some economists have scoffed at Mr. Mnuchin’s promises that tax cuts would more than pay for themselves because of the robust economic growth he says they will create. The department came under fire for removing from its web site an economic study that contradicted the secretary’s analysis of the benefits of corporate tax cuts.
Yet Mr. Muzinich’s reputation for pragmatism over ideology is providing comfort for those who know him and disagree with the direction of Mr. Trump’s agenda.
“I’m really happy that someone like Justin is working in the administration, because to have thoughtful, level-headed people who are open to evidence, and who are able to bring complicated issues together, I think leaves us better off four years later,” said Mr. Werker, a professor at Simon Fraser University in Canada, who collaborated with Mr. Muzinich on writing projects at Harvard but is not a Republican.
For now, Mr. Muzinich will be consumed with tax rates and distribution tables as he tries to help shepherd tax legislation to Mr. Trump’s desk in less than three months. Early analyses of the Republican tax plan have found that it would disproportionately benefit the rich, but the Trump administration is working with tax writers in Congress to make the legislation more progressive.
The final shape of the tax plan remains to be seen, but Mr. Taylor, who said that Mr. Muzinich was an above-average student in his Advanced Placement calculus class, is holding out hope that his former pupil will find a way to ensure that the poor are not forgotten.
“I see him in these pictures marching behind Mnuchin and I say, ‘Oh, Justin, put in a good word for us,’” Mr. Taylor, a self-described liberal, said. “I think people should be caring about other people.”