White House budget chief Mick Mulvaney has said that plans to slash social programmes are designed to increase economic growth to 3% and put "taxpayers first."
Mr Mulvaney told the House Budget Committee that he went "line by line" through the federal budget and asked "Can we justify this to the folks who are actually paying for it?"
Mr Mulvaney's appearance was one of four planned for Wednesday as Trump Cabinet officials fanned out on Capitol Hill to defend the president's budget, which contains jarring, politically unrealistic cuts to the social safety net and a broad swath of domestic programmes.
Treasury Secretary Steven Mnuchin, Education Secretary Betsy DeVos, and Agriculture Secretary Sonny Perdue are also testifying before House panels.
Panel Democrats charged that President Trump's cuts would rip apart the social safety net.
Rep Pramilla Jayapal told Mr Mulvaney that cuts to food stamps, payments to the disabled, and other programmes are "astonishing and frankly immoral".
"This budget starts by taking away health care, then food, then housing, then education, then job opportunities," Ms Jayapal said.
President Trump's budget keeps to his campaign pledge to leave Medicare and Social Security pension benefits alone and contains spending increases for the military and veterans, but it treats most of the rest of the government as fair game.
It foresees an overhaul of the tax code, which analysts say could direct most of its benefits to upper-income earners.
Mr Mulvaney also told the panel that "it'll be very difficult" to balance the budget in future proposal without cutting back the growth of Medicare and Social Security - the big retirement programmes that President Trump left alone in this year's effort.
The plan, Trump's first as president, combines 4.1 trillion US dollars (£3.2 trillion) for the upcoming 2018 fiscal year with a promise to bring the budget back into balance in 10 years, relying on aggressive spending cuts, a surge in economic growth - and a two trillion US dollars-plus accounting gimmick.
President Trump's budget is simply a proposal. There's little appetite among Capitol Hill Republicans for a genuine effort to balance the budget; GOP politicians this year are instead pressing to rewrite the tax code and forge a spending deal with Democrats that would permit higher military spending and restore Trump proposals to cut domestic agencies and foreign aid.
Many rank-and-file Republicans recoiled from the cuts, however, which would squeeze foreign aid and domestic programmes funded annually by Congress by about 10% next year and 1.4 trillion US dollars £1.1 trillion) (over the coming decade.
Mr Mulvaney, a former tea party congressman, is the driving force behind the Trump budget plan, winning the president's approval for big cuts to benefit programmes whose budgets are essentially on autopilot.