The US senate has confirmed Donald Trump's pick to run the White House budget office, giving the Republicans' tea party wing a voice in the president's cabinet.
South Carolina Representative Mick Mulvaney squeaked through the senate on a 51-49 vote.
Armed services committee chairman John McCain, who is emerging as perhaps the most vocal critic of the Trump administration, opposed Mr Mulvaney for past votes supporting cuts to Pentagon spending.
"Mulvaney has spent his last six years in the House of Representatives pitting the national debt against our military," Mr McCain said.
Mr Mulvaney's confirmation promises to accelerate work on Mr Trump's upcoming budget plan, which is overdue.
That is typical at the beginning of an administration.
But there is also the need to complete more than one trillion dollars in unfinished spending bills for the ongoing budget year, as well as transmit Mr Trump's request for a quick start on his oft-promised US-Mexico border wall and tens of billions of dollars in emergency cash for the military.
In the past, Mr Mulvaney has routinely opposed such catch-all appropriations bills, which required Republicans to compromise with former president Barack Obama, but the upcoming measure is going to require deals with Democrats.
The Republican brings staunchly conservative credentials to the post, and Trump transition officials have signalled he is likely to seek big cuts to longtime Republican targets such as the Environmental Protection Agency and other domestic programmes whose budgets are set each year by congress.
Mr Trump has indicated, however, that he is not interested in tackling highly popular benefit programmes such as Social Security and Medicare and wants a major investment in infrastructure programmes such as highways.
Democrats opposed Mr Mulvaney over his support for curbing the growth of Medicare and Social Security and other issues, such as his brinksmanship as a politician during the 2011 debt crisis in which the government came uncomfortably close to defaulting on US obligations.
"He said to me in a one-on-one meeting how he would prioritise the debts he would pay if he defaulted on the debt," said senator Claire McCaskill.
"Wouldn't that be a great addition to the chaos we are all feeling right now?"
The vote came a day after Mr Trump's pick to head the labour department, Andrew Puzder, abruptly withdrew his nomination in the face of Republican opposition.
Mr Puzder faced questions over taxes he belatedly paid on a former housekeeper not authorised to work in the United States.
Mr Mulvaney has managed to survive questions about his failure to pay more than 15,000 dollars (£12,000) in payroll taxes for a nanny more than a decade ago. He has since paid the taxes.