WASHINGTON — The Trump administration insisted on Monday, in a show of unity with one of the Republican Party’s biggest financial backers, that its plan to rewrite the tax code would be a collaborative and ultimately successful process.

In a scene that demonstrated the pressure the administration is under to deliver on that promise, Steven Mnuchin, the Treasury secretary, appeared with representatives from the political network funded by Charles G. and David H. Koch to stress that there was little daylight separating the White House, Republicans in Congress and the well-financed conservative groups that are planning to devote tens of millions of dollars to the process.

“This is a pass/fail exercise, and we will pass tax reform,” Mr. Mnuchin said in a question-and-answer session with Tim Phillips, the president of Americans for Prosperity, the Koch group that funds grass-roots political activity across the country. More than once, Mr. Phillips mentioned that the group’s activists in 36 states were ready to be deployed, and that there was no higher priority for them this year. “We’re going all in,” Mr. Phillips said.

Ideally, the administration would like a bill on President Trump’s desk by Thanksgiving.

“That, I think, is an aggressive schedule, but that is our timetable,” said Marc Short, the Trump administration’s legislative affairs director and a former executive for the Koch political organization Freedom Partners, who was also on hand for the event.