While Mr. Trump defended the call as perfectly appropriate, he confirmed that Mr. Biden came up and that he accused the former vice president of corruption tied to his son Hunter’s business activities in that former Soviet republic.

On the call: A complaint lodged by an unnamed whistle-blower details Mr. Trump’s efforts to press Mr. Zelensky to investigate Mr. Biden’s son for business dealings in Ukraine, according to our reporters’ sources.

Why it matters: U.S. security aid to Ukraine was delayed this summer, which heightened concerns that Mr. Trump could have used it as leverage and manipulated foreign policy to bolster his own re-election efforts.

Another angle: The phone call could push moderate Democrats over the fence when it comes to impeachment.