"If he decides not to spend this money, I truly am fairly confident, that, on a bipartisan basis, Congress will reappropriate it," Republican Senator Ron Johnson told reporters.
Johnson made his remarks at a news conference with Democratic Senator Chris Murphy about their recent trip to Europe, which included stops in Ukraine, Kosovo and Serbia. Murphy also visited Germany.
The two lawmakers had requested visas to visit Russia, but Moscow denied their request.
Trump administration officials said last month that the White House was reviewing whether the $250 million in military assistance for Ukraine should be sent to the country, even though it had already been approved by Congress in legislation signed into law by the president.
The money is intended for use by Ukraine in its struggle with pro-Russian separatists backed by Moscow. Russia annexed Ukraine’s Crimea region in 2014.
Johnson said he had spoken to Trump about the aid just before the trip, and Trump had told him his concern was about whether European countries, not the United States, should be providing funds to Ukraine because the country is in their "backyard."
Murphy said it was clear in meetings with Ukrainian officials that they did not have a full understanding of why the money might be withheld. He said the Ukrainians brought up the issue in every meeting with the U.S. lawmakers.
The two senators said they had sought to reassure the Ukrainians that Congress was behind the country as it faces Russian aggression.
"Regardless of what the president does, the United States Congress is with you, and we support the courage of the Ukrainian people," Johnson said.