President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's July 25 conversation with Ukrainian President Volodymyr Zelensky and the White House’s handling of the call records have ignited a firestorm on Capitol Hill, with Democrats moving full steam ahead on their impeachment inquiry.

The controversy has engulfed a host of administration officials, prompted vigorous defenses from Trump and laid the foundation for a bruising impeachment battle.

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Here are five things to know about Trump's interactions with Ukraine and the widening fallout.

Democrats zeroing in on call memo, whistleblower complaint

The controversy exploded last week with the release of two documents that provide the basis for allegations of wrongdoing against Trump.

First came a rough transcript of Trump's call with Zelensky, which was declassified and made public on Wednesday after the president said it would prove he did nothing wrong.

Democrats have seized on a few key passages, particularly one in which Trump says he would like Zelensky to do him a "favor" after discussing military aid. In another part of the conversation, Trump encourages Zelensky to "look into" Democratic presidential candidate Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE's role in ousting former Ukrainian Prosecutor General Viktor Shokin. Later, Trump tells Zelensky to get in touch with his personal attorney Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE and Attorney General William Barr Bill BarrFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Why a backdoor to encrypted data is detrimental to cybersecurity and data integrity FBI official who worked with Mueller raised doubts about Russia investigation MORE.

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The declassification and release of a whistleblower complaint on Thursday shed further light on Trump's dealings with Ukraine.

The whistleblower is a member of the intelligence community but was not on the July 25 call. Their recounting of what they had heard about Trump's conversation with Zelensky aligned with the rough transcript made public by the White House.

The whistleblower wrote that White House officials who shared information about the exchange were concerned Trump had abused the office of the presidency for personal gain.

The author of the report further alleged that administration officials sought to limit the number of people with access to the transcript of the conversation by placing it in a secure system typically reserved for matters of national security.

Barr, Pompeo, Pence among those ensnared

Barr, Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE, Vice President Pence, acting White House chief of staff Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE and former Ukraine envoy Kurt Volker have all been tied to the evolving controversy.

Trump said he would have Zelensky get in touch with Barr to discuss an investigation into the Bidens, according to the White House memo of the call.

The Justice Department has sought to distance Barr from that conversation, saying he was first notified of Trump’s conversation with Zelensky “several weeks” after it took place and that he was never asked to contact Ukraine.

Democrats have seized on Barr's involvement, calling for him to testify, recuse himself from the Ukraine matter or resign.

Mulvaney was ensnared when Trump directed him to withhold hundreds of millions of dollars of U.S. military aid for Ukraine just days before the call with Zelensky.

Pompeo, meanwhile, was reportedly on the July 25 call between Trump and Zelensky. Further complicating matters is the fact that Giuliani has indicated the State Department was aware of his efforts to dig up dirt on Biden in Ukraine.

Pompeo, one of Trump's most loyal and longest-serving Cabinet members, said last week that the State Department's dealings related to Ukraine had been "entirely appropriate."

Volker, who had been the State Department's special envoy to Ukraine, resigned last week after he was alleged to have met with Ukrainian officials to discuss Trump's desire to see an investigation into Biden.

Pence, who has made a habit of staying out of Trump-created controversies, was roped in last week when the president suggested releasing transcripts of Pence's conversations with Zelensky to prove nothing improper had taken place.

Giuliani playing key role

The former New York City mayor and personal lawyer for Trump is in many ways sharing the spotlight with Trump at the center of the Ukraine saga.

He has unabashedly pursued information in Ukraine that could be damaging to Biden, one of Trump's chief political rivals heading into 2020. Giuliani has defended his conduct, denied any wrongdoing and said he is working for his client.

Giuliani is said to have met with Shokin in late 2018 and with Shokin's successor, Yuriy Lutsenko, multiple times earlier this year. He said in May he would travel to Ukraine in search of information helpful to Trump but later called off that trip amid wide scrutiny.

Giuliani's name comes up several times in the whistleblower report, and his involvement has been a cause for concern among officials who feel he was conducting shadow diplomacy as a private citizen.

But Giuliani has been adamant in a slew of recent television interviews that the State Department was aware of his actions and had even encouraged him to take some meetings with Ukrainian officials.

Democrats are now eager to hear exactly what transpired. The House Intelligence Committee and the House Oversight and Reform Committee on Monday subpoenaed Giuliani as part of the impeachment inquiry into Trump.

Trump focusing on two Bidens

Joe Biden's son Hunter Biden joined the board of Burisma Holdings, a natural gas giant in Ukraine, in 2014.

The job came at a time when the United Kingdom, the U.S. and other governments began urging a corruption investigation into Burisma founder Mykola Zlochevsky.

Shokin, Ukraine's prosecutor general at the time, oversaw the investigation into Zlochevsky in 2015 but faced criticism for failing to do enough.

Joe Biden emerged as the point man for the Obama administration in pressing for stronger action on corruption in Ukraine as governments in the European Union and other institutions pushed for a change.

The U.S. threatened to withhold roughly $1 billion in loan guarantees if Shokin was not replaced as prosecutor general, a message Joe Biden delivered to officials in Kiev. Joe Biden recounted that conversation during a 2018 Council on Foreign Relations conference.

There's no evidence that Hunter Biden was involved in any wrongdoing at Burisma, and Joe Biden has maintained that his actions as vice president were unrelated to his son's business dealings.

But Trump and his allies have alleged that there were conflict of interest issues and have tried to paint Biden's push to oust Shokin as improper. They circulated a video of Biden's appearance at the Council on Foreign Relations last year to argue that he threatened Ukraine.

It's unclear what a Ukrainian investigation into the Bidens would yield, though Trump and his inner circle consider the matter politically damaging for the Democratic presidential front-runner.

White House, Democrats both moving swiftly

The House moved quickly last week in response to the information about Trump's call with Zelensky.

Speaker Nancy Pelosi Nancy PelosiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Health Care: New wave of COVID-19 cases builds in US | Florida to lift all coronavirus restrictions on restaurants, bars | Trump stirs questions with 0 drug coupon plan Overnight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds MORE (D-Calif.) announced the day before the rough transcript was released that the House would formalize an impeachment inquiry into the president. In the week since, the House Intelligence Committee has heard from the acting director of national intelligence, requested documents from Pompeo and sought a deposition from Volker, among others.

Democrats are expected to proceed swiftly on their impeachment inquiry with a narrow focus on the Ukraine events.

Trump, meanwhile, has responded to the latest threat to his presidency in much the same way he treated former special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's investigation. He has repeatedly proclaimed that his call with Zelensky was "perfect" and showed no wrongdoing.

He has increasingly lashed out at the whistleblower and House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.), who is leading the investigations into Trump's conduct.

The president has sought to undermine the whistleblower's credibility, seizing on the fact that they did not witness many of the events they recounted firsthand and deriding the complaint as "fake."

Trump on Monday escalated his attacks on Schiff, suggesting the lawmaker be arrested for treason for giving a paraphrased description of the July 25 call during a congressional hearing.