"Gary Cohn and Secretary Mnuchin, the Treasury secretary, and I were the three members of the troika, and what we did, earlier in the year, was said 'I'm going to focus on budget and those two guys will take taxes.' So they've really been driving the discussion on the Hill from the administration's perspective over taxes," Mulvaney said at Georgetown University's Institute of Politics and Public Service.

"So if I had to say, it was Mnuchin 45 percent, Cohn 45 percent, me 10," he added, describing how the plan was created.

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During the talk, Mulvaney said the two main features of the reform plan are lower and simpler taxes for the middle class and lower corporate taxes.

"When we lowered corporate taxes to 35 percent back in the 1980s, we were at that time, the lowest of any developed country in the world,” he said. “Now, that same 35 percent rate, which has not changed, is the highest in the developed world."

Mulvaney also discussed the challenges behind creating a new tax plan, such as developing a dynamic model to predict the new plan’s economic impact. He explained with an example from his days in the restaurant business.

"I used to sell burritos. I owned a Mexican restaurant,” he said. “I'm selling a burrito for five dollars and some people think the way to raise more money by selling burritos is to raise the price from five dollars to six dollars, and other people think the way to raise more money from selling burritos is to lower the price from five dollars to four dollars.”

He also said another problem is finding a balance between cutting taxes and not raising the national deficit.

"I got asked the other day what keeps me up at night, and it's the same thing that did when I first got here, which is the deficit," he told the crowd. "When I got here to Washington, I used to think 'Well, what we have is a revenue problem.' ... It's not a revenue problem. It's a spending problem.”

Mulvaney, who graduated from Georgetown in 1989, also offered some advice to students at the event who are pursuing a political career: "Go get a real job first. Seriously."

Earlier this week, Mulvaney said he thinks he can have a tax reform bill on Trump's desk by December.

"We felt good last week, but not only did the Senate then pass the bill, we're hearing now that the House may go ahead and take the Senate amendments or move very quickly to accept the Senate amendments and we may save as many as 10 or 12 legislative days," Mulvaney said on "Fox News Sunday."

Last week, Senate Republicans took the first step toward passing a tax plan and fulfilling a long-held campaign pledge.