Rand Paul holds up $40 billion in aid for Ukraine.

Senator Rand Paul of Kentucky on Thursday single-handedly delayed a bipartisan effort to quickly send $40 billion in aid to Ukraine, which Congress had tried to fast-track amid the escalating brutality of Russia’s war.

The Senate needed unanimous consent to waive procedural hurdles and approve the humanitarian and military aid package, which the House passed 368-to-57 on Tuesday. Mr. Paul, a Republican and a libertarian who generally opposes U.S. spending on foreign aid, objected, halting what had been an extraordinary effort to rapidly shepherd the largest foreign aid package through Congress in at least two decades.

Mr. Paul had sought to alter the bill to include a provision requiring that an inspector general monitor the spending, and was not satisfied with a counteroffer from party leaders to have a separate vote on that proposal. In his objection on the Senate floor, Mr. Paul cited concerns about inflation and rising energy and gas prices.

“My oath of office is to the U.S. Constitution, not to any foreign nation,” he said, adding, “We cannot save Ukraine by dooming the U.S. economy.”

The Senate is still expected to approve the aid package, but Mr. Paul’s objection will delay a vote until at least next week.

Any changes to the legislation would require a second vote in the House and potentially invite other lawmakers to force their own changes, delaying agreement on the legislation. Speaking on the Senate floor, Senator Chuck Schumer of New York, the majority leader, said lawmakers in both parties had concerns with the proposal.

“If every member held every bill in exchange for every last little demand, it would mean total and permanent paralysis for this chamber,” he warned. Mr. Schumer pointedly added: “When you have a proposal to change a bill, you have to convince members to support it. The junior senator from Kentucky has not done that.”

With Russia’s campaign growing more violent as the war drags into an 11th week, some Democratic and Republican lawmakers have set aside their skepticism over entangling the United States — at least financially — in a foreign war. The $40 billion package would allow President Biden to authorize the transfer of up to $11 billion of American weapons, equipment and military supplies, as well as send billions of dollars to support the Ukrainian government and refugees from the country.

Antony J. Blinken, the secretary of state, and Lloyd J. Austin III, the defense secretary, warned Congress this week in a letter that the package needed to become law before May 19 “to provide uninterrupted critical military support to our Ukrainian partners.”