WASHINGTON — President Trump is under intense scrutiny over a classified whistle-blower complaint about his behavior, which at least partly involves his dealings with Ukraine’s new president and Mr. Trump’s call for Ukraine’s government to investigate former Vice President Joseph R. Biden Jr., who is a leading candidate for the 2020 Democratic presidential nomination.

Here are some of the basic facts behind the controversy.

What did Mr. Trump do?

In a July 25 phone call, Mr. Trump is said to have pressed the president of Ukraine, Volodymyr Zelensky, to investigate Mr. Biden’s younger son, Hunter, who sat on the board of a Ukrainian energy company. Mr. Trump has seized on an unsubstantiated theory that Mr. Biden was trying to protect the company from prosecution when he called for the firing of Ukraine’s top prosecutor in 2016. Rudolph W. Giuliani, one of Mr. Trump’s personal lawyers, has pushed the Ukrainian government to investigate the matter.

Why is this coming up now?

Because of an intelligence community whistle-blower who filed a complaint last month about the president’s actions. An inspector general deemed the complaint “credible” and “urgent” and forwarded it to the acting director of national intelligence, Joseph Maguire, who has refused to share it with Congress. The issue was brought into the open when the House Intelligence Committee chairman, Representative Adam B. Schiff, Democrat of California, sent an angry letter to Mr. Maguire on Sept. 10 demanding the complaint be shared with his panel.

What did the whistle-blower claim?

The full extent of the whistle-blower’s complaint, as well as the whistle-blower’s identity, is not publicly known. Reporting by The New York Times and others has established that the complaint involves Mr. Trump’s interactions with Ukraine and a phone call with a foreign leader — possibly, but not necessarily, Mr. Zelensky. It is not clear if it includes other matters.