Health Secretary Robert F. Kennedy Jr., whose drastic overhaul of the federal health apparatus has left scientists and patients reeling, will face a demand from the Republican chairman of the Senate health committee on Wednesday to explain to Americans how his reforms “will make their lives easier, not harder.”

Mr. Kennedy will testify on Capitol Hill for the first time as health secretary, appearing back to back before the House and the Senate to promote President Trump’s budget for the Department of Health and Human Services. But he will also be asked to defend the huge reductions he has already imposed on research grants and jobs, which key Democrats have condemned as part of what they call Mr. Trump’s “war on science.”

Mr. Trump has published only the broad outlines of his budget plan, which calls for deep cuts to the National Institutes of Health and the Centers for Disease Control and Prevention. Mr. Kennedy is expected to say that the cuts will save money “without impacting critical services,” according to remarks he intends to deliver to the House Appropriations Committee.

The budget blueprint, Mr. Kennedy’s remarks say, “recognizes the fiscal challenges our country faces today, and the need to update and redirect our investments to meet the needs of a rapidly changing world.”

The remake of the health department, engineered in part by Elon Musk and his team at the Department of Government Efficiency, includes cutting 20,000 jobs — one quarter of the health work force. It also collapses entire agencies, including those devoted to mental health and addiction treatment, and emergency preparedness, into a new, ill-defined “Administration for a Healthy America.”

Senator Bill Cassidy, the Louisiana Republican and health committee chairman, is expected to call on Mr. Kennedy to articulate “a clearly defined plan or objective,” according to an excerpt from his prepared remarks. Mr. Cassidy voted to confirm Mr. Kennedy despite intense misgivings about his views on vaccines.