President Trump's July 25 conversation this summer with Ukrainian President Volodymyr Zelenskiy, flagged by a whistleblower, has sparked competing accusations between Republicans and Democrats.
The Democrats want to know whether Trump pressured Ukraine to investigate Democratic presidential candidate and former Vice President Joe Biden.
But Trump claims the real issue is Biden and his son Hunter Biden and what they did in Ukraine a few years ago.
"The one who's got the problem is Biden," Trump told reporters on Monday at the United Nations. "Biden did what they would like to have me do, except one problem: I didn't do it. What Biden did is a disgrace. What his son did is a disgrace."
The Biden camp has argued that claims of wrongdoing are unfounded and have been debunked. "Trump's doing this because he knows I'll beat him like a drum," Biden told reporters on Saturday. "And he's using the abuse of power and every element of the presidency to try to do something to smear me."
Here are five questions about the accusations and the facts behind them.
1. So what did Joe Biden do in Ukraine?
Ukraine had a revolution in February 2014, when the pro-Russian president, Viktor Yanukovych, was ousted and fled to Russia amid massive, sustained protests that focused on corruption in his administration.
A pro-Western president, Petro Poroshenko, took over. Then-President Barack Obama's administration was prepared to work with the new government, a position shared with European governments and international institutions, such as the World Bank and the International Monetary Fund. But they were all concerned about the country's endemic corruption, which had plagued the country ever since it gained independence in the 1991 breakup of the Soviet Union.
Obama made then-Vice President Biden the point man, and he became a frequent visitor to Ukraine.
2. What role did Joe Biden's son Hunter Biden play in Ukraine?
In the spring of 2014, as Joe Biden began his trips to Ukraine, Hunter Biden took a position on the board of the country's largest private gas company, Burisma. He was reportedly paid up to $50,000 a month.
The question of a possible conflict of interest — with Hunter Biden profiting in a country where his father was actively working with the government — was raised publicly at the time.
Joe Biden said that he followed government ethics regulations and that his son was a private citizen who made his own decisions.
Also, the Obama administration actually supported an investigation into the gas company because the owner, Mykola Zlochevsky, was close to the ousted president and they had both fled the country.
3. Is there any sign of wrongdoing by either Joe Biden or his son Hunter Biden?
This is what Trump and his personal lawyer, Rudy Giuliani, are suggesting. But they have not provided evidence of illegal actions. Multiple fact checks have called Trump's accusations of corruption by the Bidens misleading.
Joe Biden has actually boasted about his work in Ukraine as a spokesperson for the White House and the West generally. He called for the ouster of the top Ukrainian prosecutor, Viktor Shokin, for what was widely seen as his failure to investigate corruption.
In March 2016, Biden made one of his many trips to Ukraine and told the country's leaders that they had to get rid of the prosecutor if they wanted $1 billion in U.S. aid. Biden told the story last year at the Council on Foreign Relations:
"I said, 'You're not getting the [$1 billion]. I'm leaving in six hours. If the prosecutor is not fired, you're not getting the money.' Well, son of a b****. He got fired," Biden said.
4. How do Trump and his supporters see Joe Biden's work?
They argue that Joe Biden wanted the prosecutor ousted to protect his son from being investigated. But there has been no evidence of wrongdoing, and Joe Biden was tasked as vice president with helping to weed out corruption in Ukraine.
The key figure in pushing the corruption narrative appears to be Giuliani. He has been in contact with Ukraine multiple times and urged officials there to look into the Bidens. The story has been percolating for the past few months. Giuliani has repeatedly tweeted about it and discussed it on TV.
Meanwhile, Hunter Biden stepped down from his post at the gas company this past spring as his term expired, saying he turned down an offer to have it extended. He recognized that it was going to be an issue in the 2020 campaign.
"My qualifications and work are being attacked by Rudy Giuliani and his minions for transparent political purposes," Hunter Biden said in a statement at the time.
5. What's next?
Democrats are demanding to see a transcript of Trump's July 25 conversation with Ukraine's president, which the White House has declined to release.
The inspector general of the intelligence community, Michael Atkinson, reported the whistleblower complaint about the call to the House Intelligence Committee in a Sept. 9 letter.
But when Atkinson was called before a closed-door committee hearing last Thursday, he reportedly told members that he was being blocked from releasing the details of the complaint by Joseph Maguire, the acting director of national intelligence. Maguire has been called for the same committee this Thursday in a public hearing.
Meanwhile, congressional Democrats are talking about a new investigation of the president and whether they should pursue impeachment, a move that House Speaker Nancy Pelosi, D-Calif., has so far opposed.