President Trump repeatedly pressured Ukraine’s leader to investigate leading Democrats as “a favor” to him during a telephone call last summer in which the two discussed the former Soviet republic’s need for more American financial aid to counter Russian aggression.

In a reconstruction of the call released Wednesday by the White House, Mr. Trump urged President Volodymyr Zelensky to work with Attorney General William P. Barr and Rudolph W. Giuliani, the president’s personal lawyer, on corruption investigations connected to former Vice President Joseph R. Biden Jr. and other Democrats.

Although there was no explicit quid pro quo in the conversation, Mr. Trump raised the matter immediately after Mr. Zelensky spoke of his country’s need for more help from the United States. The call came only days after Mr. Trump blocked $391 million in aid to Ukraine, a decision that perplexed national security officials at the time and that he has given conflicting explanations for in recent days.

The aid freeze did not come up during the call, and Mr. Zelensky was not yet aware of it. Instead, he thanked Mr. Trump for previous American aid, including Javelin anti-tank weapons, and suggested he would need more as part of Ukraine’s five-year-old war with Russian-backed separatists in the country’s east.