The plans, revealed this week in documents associated with the White House budget outline, put Mr. Trump at odds with his Treasury secretary, Steven Mnuchin, who has argued that the I.R.S. needs more money and a larger staff.

Another round of cuts, tax experts say, could put one of the few federal departments that pay for themselves on life support.

“This is an agency that has had every last drop squeezed out of it,” said Dennis J. Ventry Jr., a member of the I.R.S. advisory council and a law professor at the University of California, Davis. “I don’t know how it’s going to sustain itself.”

The White House budget office has proposed a 14.1 percent cut to the I.R.S. for the fiscal year that begins in October, reducing the agency’s budget to $9.65 billion; six years ago, it stood at $12.1 billion ($13.6 billion when adjusted for inflation).