So what happened exactly?
Second, Trump wanted Ukraine to investigate one of his 2020 political rivals, former Vice President Joe Biden, and his son Hunter for his work on the board of Ukrainian energy company Burisma.
The whistleblower described his colleagues on the call as “deeply disturbed” by Trump’s demands and detailed an internal effort internally to “lock down” all records of the discussion.
Since that bombshell allegation was made public, other witnesses have corroborated the person’s account and the White House released a summary of the call that describes Trump asking Ukraine for a “favor” and to investigate the Bidens.
So what is the ‘quid pro quo’? Is it bad?
“Quid pro quo” is Latin meaning “something for something.” The term is used in employment law, for example, to prohibit sexual harassment, and under federal laws against bribery.
When it comes to foreign policy though, such negotiations are actually quite common when the U.S. demand is tied to America’s national security interests. One example is asking a country to tackle corruption before receiving U.S. assistance.
In the case of Ukraine, however, the alleged quid pro quo wasn’t about national security but about dangling U.S. money and a presidential summit at the White House in exchange for investigating a political rival.
By the time Trump had asked Ukraine’s president for a “favor” on the phone on July 25, he had already suspended nearly $400 million in U.S. military aid. Advisers to Trump wanted the aid released because it was needed by Ukraine to fend off Russian aggression. They also wanted a White House summit between the two leaders to show U.S. support for the new government.
As one Army officer assigned to the National Security Council – Lt. Col. Alexander Vindman – testified, Trump was pressuring a foreign power that desperately needed America’s help in exchange for its willingness to target U.S. citizens for the president’s personal gain.
“There was no doubt” about what Trump wanted, Vindman told Congress.
Trump has denied anything was wrong with the phone call, insisting it was “perfect.” Some Democrats, though, compare it to extortion.
OK, but is that an impeachable offense? Is it illegal?
The short answer is that it doesn’t matter if the act is illegal when it comes to impeachment.
Impeachment is a political process enshrined in the Constitution. It essentially provides a pathway for elected lawmakers to remove presidents whose actions are considered such a gross abuse of power that elected officials don’t want them to finish out their term.
And while the Constitution references “high crimes and misdemeanors,” the founding fathers didn’t necessarily interpret that to mean a violation of U.S. law. As Benjamin Franklin, in a 1787 speech put it: Without impeachment, the only “recourse” for the American public in dealing with a leader who had “rendered himself obnoxious” was assassination. And allowing for impeachment under the Constitution would give America a way to examine misconduct without resorting to violence.
On the question of whether Trump broke any laws, political campaigns are prohibited from taking anything of value from a foreign national. But the Justice Department declined to pursue the case because it didn’t have evidence that campaign finance laws were violated; officials told reporters it would have been difficult to “quantify” what Trump would have gotten from Ukraine in return.
As for facing criminal prosecution, the Constitution doesn’t say whether a sitting president can be charged in office, and the Justice Department has long had a policy that it shouldn’t happen because it would undermine the executive branch.
What’s Trump’s defense?
Trump and GOP lawmakers initially insisted there never was a quid pro quo with Ukraine and that the original whistleblower was motivated by politics. After several witnesses testified that they a presidential summit -- and possibly military aid -- was tied to Ukraine's willingness to investigate the Bidens, Republicans countered in a report released on Dec. 2 that there was no hard proof Trump was acting specifically to sway the 2020 campaign. (Trump’s intention with Ukraine would require the president or his closest aides to testify, but the White House has declined to cooperate with the inquiry.)
They also argue that Ukraine eventually received the money it wanted. While it's correct that the administration promised to release the aid on Sept. 11, by that point the White House was already alerted to the whistleblower complaint and Democrats had already announced an inquiry into what they called Trump's "Ukraine scheme."
But another key argument by Republicans could stick. They note that Democrats have been talking about impeachment since Trump took power in 2017. GOP lawmakers say the latest inquiry is just the most recent – and coordinated effort – to overturn the results of the 2016 election.
As the 2020 election inches closer, it will become harder for Democrats to argue that they need to act, rather than let voters decide at the polls.
But what about Biden? Isn’t it fair for Trump to investigate?
At the center of Trump’s allegation is a widely debunked theory that Biden – as vice president under President Barack Obama – delivered his own quid pro quo to Ukraine to help his son, Hunter, who was serving on the board of a Ukraine gas company, Burisma.
It’s correct that Biden in 2016 pressured Ukraine to fire the prosecutor general, Viktor Shokin, and threatened to withhold nearly $1 billion in U.S. aid if Shokin wasn't removed. But according to testimony by U.S. envoy Kurt Volker and others, Biden’s position was in line with U.S. policy and backed by other international powers that agreed Shokin was ignoring corruption and needed to go.
Volker also testified that it was "quite possible" that Ukrainian gas executives believed they were "buying influence" through Biden's son by putting him on the board of Burisma. The vice president's son was paid to serve on the board (at one point, his firm collected $166,666 each month from Burisma as both Hunter Biden and his associate sat on the board), and State Department official George Kent testified he warned about a perceived conflict of interest.
But no evidence has surfaced that Joe Biden acted to help his son, nor that his Hunter Biden violated any laws, and the Bidens deny any wrongdoing. House Republicans suggest in their Dec. 2 report that Democrats should acknowledge the "perception of potential wrongdoing" by Hunter Biden serving on Burisma's board while his father led U.S. policy fighting Ukrainian corruption.
So what happens next?
After weeks of both closed-door and open depositions, Democrats have moved into the next phase of the impeachment inquiry (here is a list of who testified and what they said).
On Dec. 3, Democrats were expected to release their summary of events, laying out in detail evidence that suggests the president used his power in office for personal gain and to attack a political rival.
Then on Dec. 4, the inquiry shifts to the House Judiciary Committee, which will hear from lawyers who will testify on whether Congress has grounds under the Constitution to impeach. For the week of Dec. 9, the Judiciary Committee has invited a Democratic lawyer and Republican lawyer to present their cases. It was unclear as of Tuesday whether the White House would participate.
After that the Judiciary Committee may draft articles of impeachment and vote on them, followed by a full vote in the House, which requires a simple majority.
If Trump is impeached, does that mean he leaves office? What happens next?
Impeachment doesn’t force presidents out of office. It’s more like a political black eye delivered by the House that kicks off a “trial” in the Senate.
If that happens, a group of House lawmakers -- all Democrats – would essentially act as prosecutors and argue the case before the Senate. Trump and his legal team would have equal time to put on their defense, with the Senate acting as a kind of jury.
If at least two-thirds of the nation’s senators vote against Trump, he would be removed from office – a high bar in a Senate currently controlled by Republicans. (The Senate doesn’t have the power to send a president to jail, only oust him.)
One big concern for Democrats – and a reason House Speaker Nancy Pelosi seemed to be initially reluctant to consider impeachment – is that Trump could be impeached by the House, only to be acquitted by the GOP-controlled Senate and possibly come back stronger politically as his base rallies to his defense.
That’s what happened in 1998 when Democrat Bill Clinton was impeached for lying under oath to try to hide an extra-marital affair with White House intern Monica Lewinsky. While the GOP had clear evidence of the wrongdoing, he was acquitted by the Democratic-controlled Senate and saw a boost in his approval ratings.
(President Andrew Johnson also was impeached and acquitted by the Senate in 1868).
To hear Democrats tells it, nothing less than the soul of the country is at stake.
“There comes a time when you have to be moved by the spirit of history to take action to protect and preserve the integrity of our nation. I believe, I truly believe, the time to begin impeachment proceedings against this president has come,” said Rep. John Lewis., D-Georgia, on the House floor last September. “To delay or to do otherwise would betray the foundation of our democracy.”
Republicans like Texas Rep. Mac Thornberry disagree.
“I believe it was inappropriate. I do not believe it was impeachable,” he recently told ABC’s “This Week.”
As for Trump, he says it’s nothing more than “the Greatest Witch Hunt in American History!”
ABC News’ Trish Turner and Mike Levine contributed to this report.
Editor's Note: This version corrects a reference to Clinton to note he had a boost in approval rating after impeachment, but was not up for reelection.