Trump, impeachment and Ukraine: what is going on?

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WATCH: Pelosi aims to impeach Trump over Ukraine scandal – Sep 24, 2019

A formal impeachment inquiry has been launched against U.S. President Donald Trump over allegations concerning a phone call with the leader of Ukraine about a political rival.

An impeachment inquiry — the first in Congress since the 1998 probe of then-president Bill Clinton — could lead to Trump being removed from office, although there are several steps. The Democratic-controlled House would need to vote to impeach him. Then the Republican-controlled Senate would need to remove him from office following a trial.

Here’s a breakdown of what has happened so far, why it matters and what experts have to say about it.

What has happened so far?

U.S. House Speaker Nancy Pelosi announced that the House of Representatives is launching an impeachment probe to see whether or not Trump should be impeached.

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“The president must be held accountable,” she said. “No one is above the law.”

READ MORE: Trump says call with Ukraine’s president was ‘perfectly fine’

Earlier this month, Trump came under fire after reports emerged he had used a July phone call with Ukrainian President Volodymyr Zelenskiy to call for an investigation into the activities of Biden’s son.

Trump has denied any wrongdoing. But Democrats have said the incident amounted to political pressure on Zelenskiy to find incriminating information on one of Trump’s main political rivals ahead of the 2020 U.S. election.

Additionally, Trump ordered a freeze on nearly $400 million in aid to Ukraine just days before the phone call, without citing a reason, which fueled suspicion about his motives. The funding has since been released.

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Whistleblower complaint?

Complicating matters further, the incident is the subject of an unnamed whistleblower’s complaint — one the White House has so far refused to show to Congress.

The Associated Press reported that Joseph Maguire — the acting director of national intelligence — has cited presidential privilege in his refusal to share details with American lawmakers. Maguire is scheduled to testify before the House on Thursday.

WATCH: Trump calls impeachment inquiry ‘presidential harassment’

Click to play video 'Trump calls impeachment inquiry ‘presidential harassment’' Trump calls impeachment inquiry ‘presidential harassment’
Trump calls impeachment inquiry ‘presidential harassment’ – Sep 24, 2019

Pelosi said on Tuesday that Maguire was clearly breaking the law and that he will appear before the House Intelligence Committee on Thursday.

“He must turn over the whistleblower’s complaint to the committee,” she said. “He will have to choose whether to break the law or honour his responsibility to the Constitution.”

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Two important updates on Tuesday ahead of the impeachment inquiry announcement:

The Republican-controlled Senate approved a nonbinding symbolic resolution calling for the whistleblower complaint to be released to the Senate and House Intelligence Committees.

WATCH: Impeachment: How to remove a sitting U.S. president from office

And the Democrat-controlled House is due to vote on Wednesday on a similar resolution, driving home the point that Congress is displeased with the Trump administration for preventing the release of the complaint.

The fact that the Republicans are saying, ‘You better turn this over,’ is making a lot of people say, ‘Wow,'” said University of Toronto professor Renan Levine.

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READ MORE: How the process of impeaching a U.S. president works

How are the Bidens involved?

Back when Joe Biden was vice president during the Obama administration, he led efforts to strengthen Ukraine’s democracy and tackle corruption while the country was facing Russian aggression.

At the time, Biden’s son Hunter was hired by a private Ukrainian gas company called Burisma Holdings. At the time, the Obama administration said that there was no conflict since Hunter Biden was a private citizen.

There is no evidence of wrongdoing by Biden or his son. Reuters reports that in April 2019, the younger Biden left the company.

On Tuesday, Biden said he would support an impeachment probe against Trump.

WATCH: Biden says Congress should demand ‘the information it has legal right to receive’

Click to play video 'Joe Biden says Congress should demand ‘the information it has legal right to receive’' Joe Biden says Congress should demand ‘the information it has legal right to receive’
Joe Biden says Congress should demand ‘the information it has legal right to receive’ – Sep 24, 2019

What are the latest highlights?

Trump has acknowledged the phone call. He said Monday he did nothing wrong and that he did not hold aid over Ukraine’s head for political reasons — he just didn’t want to give the country any money if there was corruption.

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On Tuesday, he said he has “authorized the release (Wednesday) of the complete, fully declassified and unredacted transcript” of the phone call.

“You will see it was a very friendly and totally appropriate call,” he tweeted. “No pressure and … NO quid pro quo!

What are we seeing happen here?

Levine made it clear that Tuesday’s announcement by Pelosi is a “historical moment.”

“This is a step that, in America’s 240-year history, this is a step that has happened four times,” he said Tuesday evening.

In an earlier interview, Levine helped place the whistleblower complaint in context.

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“If this whistleblower complaint fully comes forward and it includes what the reports say it includes,” he said, “it’s going to be very hard for, really, any Republican to want to mount a full-scale defense of the president.”

Another expert, Kori Schake — a research fellow at the Hoover Institution who has previously worked in policy roles at the White House — has said the pressure is now “really going up” on Republicans in the Senate.

I wouldn’t be surprised if this becomes the point at which Democrats in the House force Republicans to either defend the president by voting or repudiate the president,” she told Global News Washington Bureau Chief Jackson Proskow in an interview Monday. 

What protocol and presidential norms are being ignored?

Levine said this “strange” saga essentially involves two big issues around oversight and presidential power.

“It involves the improper use of political power or presidential power,” he said. ” And a refusal by the White House to comply with American checks and balances in terms of oversight of their activities.”

One issue is at the heart of the American constitution, with separation of powers, and the other issue “goes to the heart of what is appropriate for the president to be doing,” he explained.

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WATCH: U.S. Democrats react to impeachment inquiry of Donald Trump

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U.S. Democrats react to impeachment inquiry of Donald Trump – Sep 24, 2019

Schake called it “profoundly unusual.”

“It’s exactly the kind of corruption, the use of an elected office to prevent your political opposition from gaining traction, that the American founding fathers created impeachment in order to deter,” she said. 

— With files from Jackson Proskow, The Associated Press, and Patricia Zengerle and David Morgan from Reuters