The House will begin the public phase of its impeachment inquiry Wednesday with Democrats and Republicans prepared to offer competing narratives of whether President Trump inappropriately pressured Ukraine to investigate his political rivals, during televised hearings that could determine the fate of his presidency.

With separate practice sessions and in opposing memos, the two parties signaled Tuesday how they planned to present radically different interpretations of Trump’s actions and whether they were impeachable.

Democrats expressed confidence that Wednesday’s hearing would begin a serious and somber process of publicly exposing Trump’s misconduct, narrated by career diplomats who were alarmed by the president’s push to have Ukraine investigate former vice president Joe Biden and his son, as well as a debunked theory concerning the 2016 election, in exchange for military aid and a White House visit coveted by Ukraine’s new leader.

“It’s time for these witnesses to go before the American people and lay out what they saw in this extortion scheme,” said Rep. Eric Swalwell (D-Calif.), a member of the House Intelligence Committee, which will be host to the public proceedings.

In mock hearings at the Capitol, Republicans prepared to fervently defend Trump while painting the impeachment probe as a thinly veiled show trial designed to take down a president who did nothing wrong.

“We’re just making sure we’re prepared and ready to go for the hearing tomorrow,” said House Minority Leader Kevin McCarthy (R-Calif.).

The series of open hearings that begin Wednesday will be a pivotal test of lawmakers’ ability to sway public opinion for or against Trump’s impeachment in a polarized political environment where both parties are seeking to use the inquiry to their advantage heading into the 2020 presidential and congressional campaigns. It will also have the air of history — only presidents Andrew Johnson and Bill Clinton have been impeached by the House, and although neither was convicted by the Senate and removed from office, it was a defining episode of their presidencies.

The hearings Wednesday will feature testimony from William B. Taylor Jr., the acting ambassador to Ukraine, and George Kent, deputy assistant secretary of state for European and Eurasian affairs. Both men have previously told lawmakers in closed hearings that the Trump White House improperly sought to leverage an Oval Office meeting and U.S. military assistance to pressure Ukraine to open investigations into Democrats.

The proceedings offer House Democrats a chance to present what they see as incriminating evidence against Trump that they have gathered during an investigation that kicked off in September following a complaint from an anonymous whistleblower. The inquiry has included private testimony from several Trump administration officials who have said they were concerned about an effort by the president and his allies to withhold almost $400 million in taxpayer aid for Ukraine until Kyiv launched investigations that would be politically advantageous to Trump.

The aid, as well as an Oval Office meeting, were top priorities for new Ukrainian President Volodymyr Zelensky and his supporters in the United States who wanted to send a strong signal of the Trump White House’s support for Ukraine in the face of Russian military aggression.

Seeking to blunt GOP efforts to discredit the witnesses, Democrats have begun talking up the credentials of Taylor, Kent and Marie Yovanovitch, the former ambassador to Ukraine who is scheduled to testify Friday.

“Bill Taylor is a decorated Vietnam war veteran who has served his country for decades in an array of diplomatic postings,” House Intelligence Committee Chairman Adam B. Schiff (D-Calif.) said in a statement. “George Kent and Marie Yovanovitch, also career Foreign Service Officers, have spent decades in service of our country, advancing our interests and security. They will describe their own experiences and how American policy toward Ukraine was subverted to serve the president’s personal, political interests, not the national interest.”

Republicans, who have launched process-based complaints about the private hearings, have begun to shift toward substantive defenses of Trump’s behavior. In an 18-page memo, they argued that “the evidence gathered to date does not support the Democrat allegation that President Trump pressured Ukraine to investigate the President’s political rivals for his benefit in the 2020 presidential campaign.”

Republicans criticized Democrats for relying on the accounts of “unelected and anonymous bureaucrats” and argued that many of the people testifying had only secondhand information about the president’s motives. Trump himself has said he doesn’t know many of the diplomats whom Democrats have called as witnesses.

In his closed-door testimony last month, Taylor delivered a lengthy opening statement in which he methodically explained how U.S. foreign policy with Ukraine had been co-opted by Rudolph W. Giuliani, Trump’s personal lawyer, and others who sought a public declaration by Zelensky of political investigations.

Kent also said he was convinced that the president wanted Zelensky to announce investigations of Biden and Trump’s 2016 opponent, Hillary Clinton, in exchange for an Oval Office meeting.

Both men will sit before the House Intelligence Committee, as the group of 13 Democrats and nine Republicans convenes in Room 1100 of the Longworth House Office Building. Beginning at 10 a.m., Schiff and ranking Republican Devin Nunes (Calif.) will give opening statements, followed by opening remarks from Taylor and Kent.

The lawyers on the committee for the Democrats and for the Republicans will be given 45 minutes each to question the witnesses. Only Schiff and Nunes can participate in this round.

Following this portion of the hearing, the rest of the committee members will be given five minutes each to ask questions, alternating between majority and minority.

While much of the evidence in the impeachment probe is already public — from Trump’s July 25 phone call with Zelensky to text messages among key players to hundreds of pages of closed-door testimony by top administration officials — lawmakers face a critical challenge in presenting the complex case to voters through televised hearings.

Trump has already dismissed the entire process as a politically driven “witch hunt,” and his administration has blocked several key officials from providing testimony and documents.

“Democrats in Washington would rather pursue outrageous hoaxes and delusional witch hunts which are going absolutely nowhere,” Trump said Tuesday during a speech to the Economic Club of New York. “Don’t worry about it.”

On Wednesday, Trump will meet with Turkish President Recep Tayyip Erdogan at the White House and hold a news conference that could serve as counterprogramming to the live impeachment hearings.

At the heart of the impeachment probe is one chief piece of evidence: the rough transcript of the July 25 call between Trump and Zelensky, which the White House released in late September.

In the call, Trump asks Zelensky for a “favor,” pressing the Ukrainian president to investigate the Bidens and a widely debunked conspiracy theory that Ukraine, not Russia, had interfered in the 2016 U.S. election. At stake at the time was a face-to-face meeting between the two leaders and potentially $391 million in congressionally appropriated military aid to Ukraine.

To Democrats, Trump’s mention of Biden and CrowdStrike — a reference to a debunked conspiracy theory that the Democratic National Committee server hacked in 2016 ended up in Ukraine — is tantamount to catching the president red-handed as he tried to push Zelensky into conducting investigations in exchange for releasing the long-awaited security assistance and securing a White House visit.

Republicans, however, argued in their memo that the transcript of that call is “fatal” to the Democrats’ argument, positing not only that it “shows no conditionality or evidence of pressure” but also that Trump and Zelensky subsequently “have both said there was no pressure on the call.”

“It’s hard to say we should impeach the president for holding up foreign aid when the transcript never mentioned aid,” Rep. Mark Meadows (R-N.C.), a Trump ally, said Tuesday.

Plus, Republicans argue, the security assistance eventually was released, after bipartisan pressure from lawmakers, and Trump met with Zelensky in September in New York without Ukraine announcing any investigations.

The Republican document indicates that the GOP will be pursuing a dual strategy of trying to isolate Trump from charges of wrongdoing while establishing that the actions Democrats argue are impeachable — holding back aid and diplomatic engagement over concerns about 2016 election interference and Hunter Biden’s position on the board of a Ukrainian energy company while his father was vice president — were legitimate because of Ukraine’s “history of pervasive corruption.”

In his memo, Schiff issued a warning to Republicans, however, that he will not tolerate that line of defense, in his admonition that the hearings “will not serve as venues for any Member to further the same sham investigations into the Bidens or into debunked conspiracies about 2016 U.S. election interference that President Trump pressed Ukraine to undertake for his personal political benefit.”

Schiff also warned the GOP against using the process to “threaten, intimidate or retaliate against the whistleblower” — an individual whom Republicans have sought, unsuccessfully, to call as a witness in the impeachment probe.

The issue of GOP-sought witnesses, including the whistleblower and Hunter Biden, had become a sore subject for Republican lawmakers, who complained Tuesday that Schiff had yet to respond to their weekend request for witnesses who they think will help acquit Trump.

But Schiff announced late Tuesday that he would agree to some of the Republicans’ suggestions, including their request to call to testify Kurt Volker, the former special envoy to Ukraine; Tim Morrison, a former White House national security aide; and David Hale, the State Department’s third-ranking official. While these witnesses have agreed with many of the facts laid out by Taylor, Kent and others, they have played down suggestions that trump was involved in a quid pro quo.

Schiff also announced that Gordon Sondland, the U.S. ambassador to the European Union, will testify next week. After first denying in a closed hearing before lawmakers that there was any quid pro quo being sought from the Ukrainians, he later updated his testimony to say he believed there had been.

The dispute between Republicans and Democrats over what constitutes a legitimate line of argument will probably boil over into an overall public spat about the legitimacy of the impeachment inquiry, which Democrats have been defending against GOP accusations of unfairness since the process began.

Democrats spent much of Tuesday preparing for how they would tag-team with lawyers and one another, in the unique format that will govern the impeachment hearings.

“This is a time for members to take their oversized egos and put them in the foot locker,” Rep. Mike Quigley (D-Ill.) said before heading into a practice session. “This isn’t about any member; this is about the most important testimony they’re going to have here, probably in their lifetime.”

Colby Itkowitz contributed to this report.