WASHINGTON — Susan Collins was a Senate intern in 1974 when Congress, in response to President Richard M. Nixon’s refusal to spend on projects he opposed, passed a budget law to bar presidents from overriding lawmakers when it came to doling out dollars.

The resulting law, the Congressional Budget and Impoundment Control Act, is “very clear, and it reemphasizes the power of the purse that Congress has under the Constitution,” Collins, now a 72-year-old Republican senator from Maine and the chair of the Appropriations Committee, said in an interview this week.

She and her fellow appropriators in both parties will have a fight on their hands if they hope to retain supremacy in federal spending. The question of who has the final word is emerging as a central point of contention between members of Congress and the White House, a clash that is likely to escalate after the confirmation Thursday of Russell T. Vought as the director of President Donald Trump’s Office of Management and Budget.

Vought has flatly declared that he — and Trump — consider the budget act to be unconstitutional. They contend that the White House can choose what gets money and what doesn’t even if it conflicts with specific directions from Congress through appropriations measures signed into law. Others on Capitol Hill, including some Republicans, vehemently dispute that idea.

The disagreement is spurring the uproar over Trump’s move to suspend trillions of dollars in federal spending while the executive branch reviews it to determine whether it complies with his newly issued policy dictates, as well as the president’s efforts to gut the U.S. Agency for International Development.

Administration officials and many Republicans on Capitol Hill say that the president is acting within his authority and that Democrats and other critics are overreacting to a warranted and overdue attempt to scrutinize federal spending.

“There are many, many, many agencies out there, and this is going to be coming down the pike on every amount of government spending,” said Sen. Jim Risch, R-Idaho and the chair of the Foreign Relations Committee. “Elections have consequences, and this is one of the consequences: that we are going to do our best to reduce spending, do it by efficiencies.”

Democrats argue that the new administration is ignoring the law and the Constitution in usurping spending power from lawmakers and that Vought in particular is unfit for his position because of his extreme views on the authority of the executive branch. Democrats kept the Senate in overnight on Wednesday and engaged in a floor marathon assailing Vought in protest of his nomination, even though they were unable to prevent the Republican majority from confirming him.

“He’s the last person who should be put in the heart of the operation of our government,” Sen. Angus King, I-Maine, said as he warned Republicans about the dangers of ceding congressional spending power to the White House. “Once this door is open, it’s going to be very difficult to close it again no matter who the president is, no matter who is in charge.”

Serving on the Appropriations Committee has traditionally been one of the plum assignments in Congress, granting significant influence over how huge sums are spent and providing an influential perch to direct money back home. Even the leaders of the subcommittees that oversee funding specific sections of the government have such a powerful status that they are known on Capitol Hill as cardinals.

But with a new focus on cutting spending, the job has fewer rewards. Add in the prospect of a White House that ignores the directions of Congress entirely and makes its own decisions on where the dollars should go, and some lawmakers are questioning their roles. They fear that allowing its spending power to be significantly diminished would render Congress almost irrelevant.

“What’s the point of being an appropriator?” Sen. Chris Coons, D-Del. and a member of the Senate panel, asked this week.

The escalating conflict is already spilling into the lingering effort to finish off the spending bills for the current year. With many Republicans, particularly in the House, unwilling to vote for spending bills, Democrats have had to provide the necessary votes over the past two years to fund the government.

Now some are wondering why they should help majority Republicans if the White House will simply spend money the way it sees fit rather than listening to the House and the Senate.

“If you can’t trust that the president will follow the law, we have a huge problem on our hands, and that’s where we are right now,” said Sen. Chris Van Hollen, D-Md. and another appropriator.