One Republican fuming over the speech told The Hill it was an "arrogant lecture".
A Republican close to the tax-reform effort said Mnuchin has had several similar awkward moments during private meetings on tax reform.
"The better their standing on Capitol Hill, the better off the administration will be", he said.
Mnuchin also spoke favorably about Gary Cohn, the head of the president's National Economic Council. Prior to becoming Donald Trump's treasury secretary, he was a partner at Goldman Sachs, started a hedge fund named for a spot near his house in the Hamptons, and ran OneWest, a mortgage lender that foreclosed on more than 36,000 homeowners during Mnuchin's tenure.
"They bring policy expertise to the table within their agencies". "We must do this without adding to our staggering debt", says West Virginia Democrat Joe Manchin, while affirming that he himself is fully in favor of pushing for a reduction of taxes in the US.
The answer, we now know, is no, but not because Mnuchin didn't try.
Many GOP lawmakers are hungry for details about the upcoming tax plan.
Mncuhin is an independently wealthy Wall Street banker.
Republicans across the board want to lower the corporate rate, which is now 35 percent, as part of tax reform.More news: Utilities in US Southeast restore power to almost half hit by Irma
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Underscoring the president's desire for tax legislation, Trump was hosting a bipartisan group of senators for dinner at the White House on Tuesday, including a trio of moderate Democrats from states Trump won last November and whose votes he'd like to have on a tax bill.
This poses a problem for Republicans, as there is now no such tax proposal in Congress.
The tax overhaul principles Republicans have put forth envision slashes in the corporate tax rate and individual tax rates. Without a budget resolution there is no tax reform - and Republicans have to agree on a 2018 budget resolution in order to use reconciliation to pass a tax plan in the Senate with only 50 votes rather than 60.
His relationship with Trump is strained, people close to both men say, but he has built a strong rapport with some members on Capitol Hill who view him as a competent partner as tax reform moves to the top of the agenda.
The source said it was jarring to see the treasury secretary appear alongside a House staffer.
Mnuchin has also had a rough several weeks.
Mnuchin and his wife, actress Louise Linton, were criticized last month, when they traveled to Kentucky, where he participated in an event at Fort Knox, on a government plane.
Treasury Secretary Steven Mnuchin asked for a costly government plane to fly to his European honeymoon earlier this year.