The complaint cites Ms. Yovanovitch’s ouster as one of a series of events that paved the way for what the whistleblower alleges was an abuse of power by the president. State Department officials were told this spring that Ms. Yovanovitch’s removal was a priority for the president, a person familiar with the matter said. Secretary of State Mike Pompeo supported the move, an administration official said. Ms. Yovanovitch was told by State Department officials that they couldn’t shield her from attacks by the president and his allies, according to people close to her. In an interview, Mr. Giuliani told The Wall Street Journal that in the lead-up to Ms. Yovanovitch’s removal, he reminded the president of complaints percolating among Trump supporters that she had displayed an anti-Trump bias in private conversations. In Mr. Giuliani’s view, she also had been an obstacle to efforts to push Ukraine to investigate Mr. Biden and his son Hunter.

As vice president, Mr. Biden spearheaded an international anticorruption reform push in Ukraine, which included calling for the dismissal of a prosecutor the U.S. 50,000 a month, according to one official with ties to the company. Mr. Trump has accused the Bidens of corruption. In May, Ukraine’s then-prosecutor general, Yuriy Lutsenko, said he had no evidence of wrongdoing by the Bidens. When Ms. Yovanovitch left her post in May, the State Department said she was concluding her assignment "as planned," and that her departure date aligned with the start of a new administration in Ukraine. She was recalled at least three months before the end of the customary three-year diplomatic tenure. Mr. Giuliani said he gave Mr. Pompeo a nine-page document dated March 28 that included a detailed timeline of the Bidens’ dealings in Ukraine and allegations of impropriety against Ms. Yovanovitch, including that she was "very close" to Mr. Biden. "He called me back and he said they were going to investigate," Mr. Giuliani said of the secretary of state, saying Mr. Pompeo asked for additional documents to back up the allegations.

Neither the State Department nor the White House responded to requests for comment. Andrew Bates, a Biden campaign spokesman, said Mr. Biden has professional respect for Ms. Yovanovitch but that the two aren’t close. "She became our ambassador during the final 6 months of the administration," he said. "This is standard Rudy Giuliani: noun, verb, lie about Joe Biden. When asked about Ms. Yovanovitch’s removal Thursday, Mr. Trump told reporters: "I don’t know if I recalled her or somebody recalled her but I heard very, very bad things about her for a long period of time. Ms. Yovanovitch couldn’t be reached for comment. She is set to testify before House lawmakers on Oct. 11 as part of the impeachment inquiry. People close to her disputed that she did anything wrong and defended her work. "She was doing everything by the book," said a senior Ukraine government official who interacted with her. Ms. Yovanovitch remains an employee of the State Department and is a senior State Department fellow at Georgetown University.

Prior to Ms. Yovanovitch’s recall from Kyiv, her relations with some senior Ukrainian officials were fraught. Ms. Yovanovitch openly criticized the office of Mr. Lutsenko, then the prosecutor general, for its poor anticorruption record. "Lutsenko hated her because she pushed for reforms, especially in the judiciary sector," said a former Western diplomat in Ukraine. Presidents have the authority to nominate and remove ambassadors. But some senior officials at the White House and State Department say they had been unaware of the president’s displeasure with Ms. Yovanovitch and surprised by her removal. Mr. Giuliani’s role in pressing for the ambassador’s ouster is unusual given that he holds no formal government role. The president’s critics contend that, in his capacity representing the president’s personal interests as his attorney, he has exercised undue influence over administration policy and personnel. Mr. Giuliani isn’t the only figure outside the administration to have expressed concerns about the ambassador. Mr. Sessions told the Journal he didn’t follow up on the matter and didn’t hear until months later about Mr. Trump’s interest in replacing her.

He declined to say where his information about the ambassador came from but said his letter was in line with a broader concern among members of Congress that the administration wasn’t moving swiftly enough to put new ambassadors in place. Should career diplomats reflect a president’s political interests or be shielded from them as they represent the U.S. Join the conversation below. In a March 2019 interview with a columnist at The Hill, Mr. Lutsenko complained that the U.S. Embassy in Kyiv was obstructing corruption investigations, including by providing a "do not prosecute" list and restricting Ukrainian access to the U.S. Mr. Lutsenko’s claim is mentioned in the whistleblower complaint. The U.S. State Department at the time called the untouchables list claim an "outright fabrication." Mr. Lutsenko later retracted the allegation about the list and said had no evidence of Biden wrongdoing. He was dismissed in August. In early 2019, Mr. Lutsenko met twice with Mr. Giuliani, who around the same time stepped up his quest to collect information he could use to persuade Ukraine to open an investigation into the Bidens. The men met in New York in January and in Warsaw in February.