House Speaker Nancy Pelosi and her lieutenants are turning a new page in their impeachment inquiry this week based on a principle familiar to classics scholars: repetitio mater studiorum.

"Repetition is the mother of all learning."

For news audiences, key details about the Ukraine affair have been told, so far, twice: First, in leaked and preliminary accounts of what witnesses told investigators behind closed doors, then in the full transcripts released last week of their depositions.

This week, some witnesses will tell their stories for a third time, in open hearings scheduled before the House Permanent Select Committee on Intelligence.

On Wednesday, lawmakers are scheduled to hear testimony from two diplomats, William Taylor, the acting U.S. ambassador to Ukraine, and George Kent, the deputy assistant Secretary of State for European and Eurasian affairs.

On Friday, the committee is scheduled to hear from the former U.S. ambassador to Ukraine, Marie Yovanovitch, who was recalled from her mission to Kyiv this spring in an important early stage of the Ukraine affair.

Pelosi, Intelligence Committee Chairman Adam Schiff, D-Calif., and others believe they've now established the broad facts of the case, having confirmed and re-confirmed them from a number of witnesses.

Now they want some of the key actors to tell their stories once more in open testimony to try to shape public opinion.

The case

Many of the underlying facts aren't in dispute, including, in some instances, by the White House itself.

President Trump authorized his personal lawyer Rudy Giuliani to pursue a parallel foreign policy in Ukraine separate from that being run by the professional national security and diplomatic experts within the government.

The White House sought to involve a few administration officials whom it considered reliable in that parallel policy, while excluding others it apparently didn't — including Yovanovitch and Kent.

The object of these arrangements, according to what witnesses have said, was a strategy to extract concessions from Ukraine's president, Volodymyr Zelenskiy.

Trump would agree to a White House meeting and to the continued flow of military assistance if Zelenskiy committed in public to investigating a conspiracy theory about the 2016 election interference and the family of former Vice President Joe Biden.

What Trump wanted was "nothing less than President Zelenskyy to go to microphone and say 'investigations, Biden, and Clinton,'" Kent told House investigators.

Another diplomat, Gordon Sondland, the U.S. ambassador to the European Union, told Congress in an addendum to his original testimony that he made clear to a top Zelenskiy aide that engagement and military assistance depended on the public commitments that Trump wanted.

Fiona Hill, the former top Russia specialist on the National Security Council, described a key July 10 meeting at the White House in her deposition with House investigators.

Ukrainian officials at that conference pressed for a Zelenskiy meeting with Trump. Then-National Security Adviser John Bolton wouldn't commit, per Hill. But Sondland said he already had arranged a meeting with acting White House chief of staff Mick Mulvaney — if the Ukrainians agreed to investigations.

Bolton ended the meeting, Hill said, and she also described him declaring he wanted no part of any "drug deal" involving political investigations.

The White House froze about $391 million worth of Ukraine military assistance in the summer even after it had been approved by the Defense Department and other agencies. The money was then released in September.

Republicans' defense

Trump has described his July 25 call with Zelenskiy as "perfect" and says there was nothing wrong with him seeking to quash what he called "corruption" in Ukraine. His supporters, meanwhile, have offered parallel defenses.

House Republicans, led by key Trump allies including Rep. Mark Meadows of North Carolina and Rep. Jim Jordan of Ohio, argue this is a hearsay case.

None of the diplomats who've described the Ukraine affair heard directly from Trump or another principal, such as Mulvaney, about why and how they were acting, the Republicans say.

And when Sondland did phone Trump directly to ask him about the object of the policy he and others were carrying out, Sondland told members of Congress that the president said he wanted "nothing" from Ukraine — "no quid pro quo."

That defense was complicated by Sondland's revised account of his original testimony — and by Mulvaney's statement that all foreign policy is political and that the people in the administration work for the president.

It's their job to carry out his instructions, Mulvaney said, and everyone needs to "get over it."

The ineptitude and broad powers defenses

A few of Trump's supporters in the Senate have offered different defenses for the Ukraine affair.

Sen. Lindsey Graham, R-S.C., observed that Zelenskiy never made the public statement that Trump wanted and accordingly there was no exchange of favors. Moreover, the White House released the frozen military assistance.

Graham argued the Trump administration was too incompetent to actually bring about a quid-pro-quo, and no president should be impeached for ineptitude.

Other defenders have taken a bigger-picture view: Sen. Rob Portman, R-Ohio, said that he believes it's improper for an American president to invite foreign assistance into a U.S. political campaign — but not impeachable.

House Minority Leader Kevin McCarthy, R-Calif., meanwhile, has called all of Trump's conduct in the Ukraine case "legitimate" and within his authority.

These and other arguments likely would be aired if the impeachment case reaches a trial in the Senate.

Twenty Republicans would have to break ranks and vote with all Democrats to reach the constitutionally required two-thirds majority, or 67 votes, to convict and remove the president from office.

As things stand, Trump's red wall in the Senate appears likely to protect him.

The trial

Impeachment is the equivalent of a grand jury indictment, in which House lawmakers state whether they believe there is sufficient evidence of " high crimes and misdemeanors" to prompt a trial in the upper chamber.

Pelosi and her other deputies, including House Judiciary Committee Chairman Jerry Nadler, D-N.Y., have said that impeachment isn't a foregone conclusion and they must, as Nadler put it, "keep an open mind — and the appearance of an open mind."

Even so, only two House Democrats opposed the legislation formally initiating the impeachment inquiry. Some members have been talking about impeachment since not long after Trump was inaugurated.

Congress cannot stand by and abide what Trump has done, Pelosi argues, because if it does not respond to his actions in the Ukraine affair, its status as a check on the executive branch would become meaningless.

What isn't precisely clear yet is how much longer it may take for the House to complete its work and set up the next phase in the Senate. Pelosi and Nadler have been unwilling to state explicitly that they believe the House could vote on articles of impeachment by Christmas.

The process must take as long as it takes, Nadler said.

What is certain is that Senate Majority Leader Mitch McConnell, R-Ky., a key ally of the president, has begun to make plans for the Senate to spend several weeks on a trial. If House Democrats send over articles of impeachment, he said, Senate Republicans will take them up.

"We intend to do our constitutional responsibility," McConnell said.

Copyright 2019 NPR. To see more, visit https://www.npr.org.