As European Union leaders meet in Brussels for their quarterly summit with a goal of securing new financial and political support for Ukraine, Prime Minister Viktor Orban of Hungary is the one man holding up both.

Mr. Orban, who has in the past held up E.U. sanctions against Russia and is seen as President Vladimir V. Putin’s closest ally in the bloc, said on Thursday that Ukraine was not ready to start negotiating membership in the alliance. He also said that 50 billion euros, about $52 billion, in proposed aid for Ukraine should come only after Europe-wide elections planned for the summer.

The two-day E.U. summit is being held at a crucial moment for Ukraine: Its president, Volodymyr Zelensky, just held bruising meetings in Washington, where he was unable to secure desperately needed money for his war effort because of political divisions in Congress. The E.U. aid would be a major boost, just as good news on at least opening formal negotiations on Ukraine’s membership prospects could turn around a souring atmosphere, even as Mr. Putin declared that Russia’s aims in the war were unchanged.

As Mr. Orban arrived in Brussels for the summit, he said: “Enlargement is not a theoretical issue; enlargement is a merit-based, legally detailed process, which has preconditions.” He claimed Ukraine had not fulfilled the formal criteria to open talks about membership.