Rudy Giuliani, President Trump’s personal attorney, has called off a trip to Ukraine during which he planned to ask the country’s prosecutors to investigate the origins of the Mueller investigation and to look into ties between Hunter Biden, Democratic presidential candidate Joe Biden’s son, and a Ukrainian oligarch.

With respect to Biden, Giuliani was particularly interested in a phone call Joe Biden made while serving as vice president in which he requested Ukraine’s top prosecutor, who had once been tasked with investigating a company Hunter Biden worked for, be removed from office.

Giuliani’s decision came amid growing criticism that he was requesting that a foreign state interfere in the 2020 election. A number of polls show Joe Biden is currently the Democratic frontrunner, and he could become Trump’s rival in the next election. A scandal could obviously hurt Biden’s chances not only in a hypothetical general election contest, but in the primary.

Democrats and some legal experts were quick to attack Giuliani and his trip. Sen. Chris Murphy (D-CT) called the trip “next level” corruption, and House Judiciary Committee Chairman Rep. Jerry Nadler (D-NY) said, “We’ve come to a very sorry state when it’s considered okay for an American politician — never mind an attorney for the president — to go and seek foreign intervention in American politics.”

I know we’ve become numb to the corruption of this Administration, but this story should be the total focus of the political class today. The President is openly asking a foreign government to investigate his political rival. This is next level. https://t.co/pboUJuCPoj — Chris Murphy (@ChrisMurphyCT) May 10, 2019

Tim Meyer, an international law expert at Vanderbilt University, told the Washington Post the trip set a shocking precedent. “This is the first instance of which I am aware in which a private lawyer for the president of the United States has, in his own words, ‘meddled’ in a foreign criminal investigation of a third party in order to politically benefit the president.”

Ahead of the trip, Giuliani argued for asking for Ukrainian help on Fox News, where he claimed that there was “massive collusion” between Democratic National Committee officials, Clinton campaign representatives, the Obama administration, and Ukrainian politicians, who he said lied about Paul Manafort and Donald Trump.

“It’s a big story,” Giuliani said. “It’s a dramatic story and, I guarantee you, Joe Biden will not get to election day without this being investigated. Not because I want to see him investigated, this is collateral to what I was doing.”

Manafort, Trump’s former campaign manager, was sentenced to a total of 7.5 years in federal prison last March. As Vox’s Andrew Prokop reported at the time:

All the charges brought against Manafort by prosecutors related to his past political and lobbying work for Ukraine’s pro-Russian political faction, his finances, or attempting to interfere with the investigation. He has not been charged with any criminal conspiracy to interfere with the 2016 elections.

Manafort was subsequently indicted on 16 additional charges in New York, where he is not eligible for a presidential pardon.

The origins of a Republican Ukrainian conspiracy theory

Giuliani hoped to prove that Ukrainian officials had helped the Clinton campaign in 2016 the same way the Russians helped the Trump campaign. Part of this help supposedly came in the form of documents about Manafort’s Ukrainian ties.

Accusations that Ukraine worked on behalf of Clinton have been floated, particularly on the right, following a 2017 report that a Ukrainian American contractor with the Democratic National Committee (DNC) passed along information about Manafort from Ukrainian officials to members of the Clinton campaign. That contractor, Alexandra Chalupa, told CNN she did give the DNC information about Manafort, but only what she knew about the man from her work as an activist.

“When it was announced that the Trump campaign hired Manafort, many Ukrainian Americans were alarmed and concerned it was an early signal that Putin was trying to influence the US election,” Chalupa said. “At that time, I flagged for the DNC the significance of his hire based on information in the public domain.”

A number of DNC officials involved in opposition research have disputed the claim that the Clinton campaign ever reached out to Ukraine, as have representatives from the country itself. Nevertheless, Giuliani believes there is more to the story.

“All I want the Ukrainian government to do is investigate, and don’t let these people buffalo you,” the president’s lawyer said on Fox.

Giuliani also believes Joe Biden may have acted inappropriately while serving as vice president.

In 2016, Biden threatened to withhold $1 billion from Ukraine unless the country made some anti-corruption reforms, including removing Ukraine’s top prosecutor, from office. As Bloomberg’s Stephanie Baker and Daryna Krasnolutska reported, the threat was made in part to aid a UK investigation into money laundering by Mykola Zlochevsky, the head of Burisma Holdings. The former vice president’s son, Hunter Biden, joined Burisma’s board in 2014; in that role, he was reportedly paid $50,000 a month.

The UK’s investigation into Zlochevsky was underway by the time Hunter Biden joined his company, and Ukraine began an investigation into the mogul shortly after the UK launched its inquiry.

According to an official Bloomberg spoke with, and documents Baker and Krasnolutska reviewed, both investigations into Zlochevsky had been halted by the time Joe Biden made his call.

A British court unfroze the mogul’s assets in 2015 after prosecutors struggled to make their case against him; they blamed a lack of cooperation from their Ukrainian counterparts. The Ukrainian investigation, which was being handled by the office of the country’s prosecutor general, was put on hold in 2014. Viktor Shokin, who at the start of the investigation was a deputy prosecutor in the office, and who had become prosecutor general by the time Biden made his call, has been accused by subordinates of blocking the investigation into Zlochevsky. He has also been accused of corruption.

Although Biden has not recently spoken about the call or his son’s work in Ukraine, he did in 2015 tell Bloomberg, “No one has ever raised that with me in Ukraine,” and that, “I don’t talk to my son” about what he does for a living.

Despite apparent issues with the timing of Biden’s call, Giuliani hoped his trip would reveal some wrongdoing, and said so in response to Sen. Murphy’s criticism.

Explain to me why Biden shouldn’t be investigated if his son got millions from a Russian loving crooked Ukrainian oligarch while He was VP and point man for Ukraine. Ukrainians are investigating and your fellow Dems are interfering. Election is 17 months away.Let’s answer it now https://t.co/FT34kX7Pst — Rudy Giuliani (@RudyGiuliani) May 10, 2019

While Giuliani’s trip has been cancelled, the issue of Joe Biden and Ukraine may yet remain a subject of discussion in the White House.

In an interview with Politico Friday, when asked whether he planned to task Attorney General William Barr with investigating the Bidens, President Trump signaled a willingness to do so.

“Certainly it would be an appropriate thing to speak to him about, but I have not done that as of yet,” Trump said. “It could be a very big situation.”