Since President Trump took office vowing to pull back U.S. support for Ukraine, European leaders have worried that they would be unable to supply Ukraine with the weapons it needs.

So far, it looks like they were right.

The so-called coalition of the willing of European nations backing Ukraine has struggled to get materiel to its battlefields in the time since Mr. Trump made clear that Europe needed to shoulder more of the load for Ukraine’s security and its own.

That is one reason Ukraine’s Parliament overwhelmingly approved on Thursday a deal to give the United States a share of future revenue over natural resources, including minerals. While short of a security guarantee, it keeps open the possibility of continued shipments of American arms and other military assistance.

“This gives us hope,” said Yehor Chernev, the deputy chair of the Ukrainian parliament’s committee on national security, defense and intelligence.

In an interview shortly after the vote, Mr. Chernev said Ukrainian forces were running low on long-range missiles, artillery and, above all, ballistic air defense systems — the majority of which are manufactured in the United States, according to an analysis by the Kiel Institute for the World Economy.

By summer, military aid approved under the Biden administration will run out, and Mr. Trump appears reluctant to renew it.