A handful of Senate Republicans joined Democrats on Wednesday in sharp questioning of President Trump’s proposed budget cuts, exposing the depth of congressional unease with the White House’s new plan to pare back billions of dollars for foreign aid and public broadcasting.

The rare display of bipartisan discord left the fate of that package uncertain at a moment when the Trump administration has signaled that it is willing to circumvent Congress to slash federal spending, potentially touching off a constitutional battle over the power of the purse.

The dynamic played out over a tense, roughly three-hour grilling of Russell T. Vought, the White House budget director, who asked lawmakers to approve Mr. Trump’s request to rescind more than $9 billion in enacted funds. The administration has framed the package, unveiled this month, as the first of possibly many that could implement changes identified by the Department of Government Efficiency, or DOGE.

But Democrats and some Republicans on Wednesday questioned the president’s proposed clawbacks, which passed the House earlier in June. Some lawmakers said the cuts would undermine longtime bipartisan priorities, including a shared desire to preserve local television and radio stations and combat the global AIDS crisis.

Senator Susan Collins, Republican of Maine and the chairwoman of the Senate Appropriations Committee, said she worried about the implications for global health, particularly because some of the funding that the president targeted has “saved more than 26 million lives.”

Lawmakers from both parties later echoed some of those criticisms, prompting Ms. Collins to conclude the hearing by saying that it showed the “depth of concerns about this rescission from members on both sides of the aisle” with the White House’s plans. A spokesperson for the senator later confirmed that she was drafting an amendment to change the package when it reached the Senate floor.

Democrats took more direct aim at Mr. Vought, in a series of combative exchanges that challenged not only the proposed cuts but also the president’s broader views about his own spending powers.

At one point, Senator Patty Murray of Washington, the top Democrat on the panel, pressed Mr. Vought to say whether the White House would try to strike federal funds even if Congress did not approve, an idea that the budget chief has raised publicly in recent days.

Mr. Vought declined to rule out that possibility and defended his tactics and legal reasoning. He stressed repeatedly that there were other ways for Mr. Trump to cut spending, in comments that suggested the White House could try to sidestep a vote in Congress.

“We believe we have, under the law, numerous options with regard to how to achieve savings,” Mr. Vought said.

The tense clashes served to illustrate the stakes in Mr. Trump’s vast, chaotic and ongoing reorganization of American government. The president has sought to eliminate trillions of dollars in federal spending across a stunning array of education, health, housing, labor and science programs, describing many of these once-core federal functions as “woke” or wasteful.

At the heart of that effort is Mr. Vought, now in his second stint as White House budget director. An avowed conservative with deeply held Christian views, Mr. Vought espouses an expansive theory of presidential power. His approach has sparked a wave of lawsuits and dozens of ongoing federal investigations into the White House and its attempts to cut spending that does not align with the president’s political agenda.

Those tactics loomed large as the powerful Senate Appropriations Committee convened to consider Mr. Trump’s request to cancel roughly $9 billion previously authorized by Congress. Using the power of rescission, Mr. Trump may freeze a set of targeted funds for 45 days, then ask lawmakers to cancel the money. The spending must be restored if the House and Senate do not approve the request.

Mr. Trump’s proposal would slash previously promised funding for the Corporation for Public Broadcasting, which funds NPR and PBS, while eliminating a vast array of money allocated to the shuttered U.S. Agency for International Development. The cuts in foreign aid include a $400 million reduction to the President’s Emergency Plan for AIDS Relief, a bipartisan program known as PEPFAR.

On Wednesday, Mr. Vought said the administration had targeted funding that might have had “benevolent sounding titles” but reflected government waste, saying that an affirmative vote would show that Congress was “serious about getting our fiscal house in order.”

But Ms. Collins and Ms. Murray each raised concerns that the White House request would undermine the bipartisan funding deal that they clinched only months earlier, which Mr. Trump signed into law to avert a government shutdown. The two lawmakers also maintained that the Trump administration had failed to provide a thorough accounting about how some of the cuts would be applied.

“We can’t tell, in looking at the information you’ve given us — because it’s not specific — whether the rescission would harm our efforts to prevent the spread of tuberculosis, or polio, or malaria,” Ms. Collins said.

Mr. Vought maintained throughout the hearing that “no lifesaving treatment will be impacted.”

But his reassurances did not always satisfy members from his own party.

Senator Mike Rounds, Republican of South Dakota, expressed alarm about the nation’s $36 trillion debt and said he supported efforts to “rein this in.” But he also said he feared that the president’s cuts to public broadcasting would mean that some local stations in his state would “not continue to exist.”

Senator Mitch McConnell of Kentucky, the former Republican leader, launched a broadside against the Trump administration for cuts to foreign aid that would undermine American “soft power,” allowing foreign adversaries including China to fill the gaps.

Senator Lisa Murkowski, Republican of Alaska, offered a more general plea to lawmakers, urging her colleagues to show “fidelity to our requirements under the Constitution” to set the nation’s spending levels.

Still, most Republicans refrained from any direct challenge to Mr. Trump, his leading budget aide and their broad claims of executive power. Senator Lindsey Graham of South Carolina even suggested he would support the package, despite his past support for PEPFAR, saying that “the way you run the government has consequences.”

“Don’t lecture me about being mean or cruel,” Mr. Graham said.

Mr. Vought tried to frame the proposed cuts as a response to wasteful spending — and a reflection of an annual budget-setting process that he described as “broken.” He also signaled that the White House did not plan to back down from seeking to cut spending unilaterally if Congress failed to act, stoking Democrats’ ire about a potential abuse of power.

“We have the power of the purse,” stressed Senator Kirsten Gillibrand, Democrat of New York. “I don’t know why more Republicans aren’t standing up to President Trump and these very ill-advised decisions he’s making, because it’s harming their constituents.”

In the days before the Senate convened its hearing, Mr. Vought had signaled that the administration could try to use federal law to its advantage by waiting to transmit a rescission request until closer to the end of the fiscal year, which concludes on Sept. 30.

Even if Congress does not vote on the proposal, the president’s action would freeze the money for so long that it would ultimately expire anyway, a little-known and legally untested maneuver that Mr. Vought has described as a pocket rescission.

“What’s at stake here is more than the particular provisions of the rescission package,” Senator Brian Schatz, Democrat of Hawaii, said earlier in the hearing. He said it would be up to lawmakers to decide whether they would “willingly set up a situation where bipartisan negotiations are ripped up” whenever one party controls Congress and the White House.

Megan Mineiro contributed reporting.