The whistleblower complaint at the heart of House Democrats impeachment inquiry into President Donald Trump focuses on a July phone call he had with Ukraine's president. What is the controversy about?

The July 25 phone call

When Trump spoke with Volodymyr Zelenskiy, the Ukraine's newly elected president was eagerly awaiting some $400 million in U.S. aid approved by Congress. During the call, Trump asked the president to investigate corruption involving former Vice President Joe Biden's son, Hunter, who was working for a Ukrainian energy company that was once under investigation by Ukrainian prosecutors.

"There's a lot of talk about [Joe] Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the [Attorney General] would be great," Trump said, according to a call memo released by the White House.

Memorandum of a Telephone Conversation: President Trump's Telephone Conversation with Ukrainian President Volodymyr Zelenskyy President Donald J. Trump released a declassified transcript of his telephone conversation with Ukrainian President Volodymyr Zelenskyy from July 25th, 2019.

Trump believes Biden tried to have Ukraine's prosecutor fired in 2015 to protect his son from a pending prosecution. Ukraine political observers say there is no truth to the allegation that Hunter Biden, or the Ukrainian natural gas company he worked for, were facing any sort of legal jeopardy, and that the prosecutor Biden helped to force out, Viktor Shokin, was widely regarded as a failure.

The controversy

The whistleblower who alerted lawmakers to the call said "multiple U.S. officials with direct knowledge of the call" said that the president used the conversation to advance his personal interests, primarily investigating his political opponent Joe Biden and Biden's son. The whistleblower claimed White House officials he spoke with "were deeply disturbed" because of the possibility that "they had witnessed the President abuse his office for personal gain."

Was the call about cracking down on corruption?

In the days since the whistleblower complaint went public, Trump has defended the call as "perfect" and "appropriate" because he maintains he was pressing Ukraine's new leader to crack down on corruption.

In remarks to reporters at the White House on Oct. 3, the president elaborated on his Ukraine request and expanded it to include an investigation into Biden's son's business ties in China.

"I would think that if they [Ukrainian officials] were honest about it, they'd start a major investigation into the Bidens. It's a very simple answer. They should investigate the Bidens ... and by the way, likewise, China should start an investigation into the Bidens. Because what happened to China is just about as bad as what happened with Ukraine," Trump said.

The president's critics reject this defense, and say the call clearly shows the president asking a foreign official for a favor that would help him politically. Those critics include the current head of the Federal Election Commission, who cited the law that says U.S. officials cannot solicit a "thing of value" given to affect an election.

Was the phone call about a political favor?

Opposition Democrats are conducting hearings to determine what U.S. diplomats were telling Ukrainian officials about U.S. policy in recent months, and whether the president was making U.S. aid for Ukraine contingent on whether Kyiv would investigate the Bidens.

Senator Mitt Romney is one of only a few sitting Republicans in Congress supporting further questions into the call.

Democratic Representative Adam Schiff, who is leading the House impeachment inquiry, has said there is broad consensus against accepting any foreign election assistance.

Investigations multiply

While Democrats press for more information into the administration's Ukraine policy and the infamous phone call, the president's political allies are calling for broader hearings into Ukraine.

Senator Lindsay Graham has invited the president's lawyer, Rudolph Giuliani, to brief the Senate Judiciary Committee, meaning the July phone call and U.S. Ukraine policy will remain at the center of a political storm and at the heart of the president's political future.