The impeachment inquiry of U.S. President Donald Trump shifts into higher gear this week when a parade of officials will face questioning by Democratic lawmakers seeking details that could link Trump to a pressure campaign against Ukraine potentially for his political benefit.

Eight more witnesses are due to testify in Week Two of the televised hearings. They include Gordon Sondland, the US ambassador to the European Union, whose direct interactions with Trump are likely to be the main focus in the investigation of whether the president made security aid to Ukraine contingent on it agreeing to dig up dirt on a political rival, former Vice President Joe Biden.

The latest round of hearings will stretch from Tuesday to Thursday before the House of Representatives Intelligence Committee. Seeking to build on last week’s testimony by three key witnesses, Democrats leading the inquiry – the first public impeachment drama in two decades – will continue trying to make the case that Trump abused the power of his office.

Denying any wrongdoing, Trump, who railed on Twitter and elsewhere against the proceedings and attacked witnesses by name last week and over the weekend, has shown no sign of a let-up in his confrontational approach. Some Democrats have accused him of witness intimidation but most Republican lawmakers have joined him in declaring the inquiry unfair.

Several witnesses testified last week that they were alarmed over the pressure tactics used against Ukraine, as well as the role of Trump’s personal lawyer, Rudy Giuliani.

At the heart of the inquiry is a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to open a corruption investigation into Biden and his son, Hunter Biden, and into a discredited theory that Ukraine, not Russia, meddled in the 2016 US election.

Among this week’s witnesses are several who listened in on the call.

Lieutenant Colonel Alexander Vindman, a Ukraine expert at the National Security Council, testified behind closed doors last month that he was so concerned about efforts to push Ukraine to investigate a Trump rival that he reported it to the NSC’s lawyer.

Fiona Hill, Trump’s former top Russia adviser, who also testified in private last month and is now set to appear on Thursday, previously recounted how US policy on Ukraine got caught up in clashes between official and unofficial channels.

Due to testify on Tuesday are Kurt Volker, former US special envoy to Ukraine; Jennifer Williams, an adviser to Vice President Mike Pence; and Tim Morrison, an NSC aide. Wednesday will see Laura Cooper, the deputy assistant secretary of defense for Russia, Ukraine and Eurasia, and David Hale, the undersecretary of state for political affairs, at the witness table.

The hearings could pave the way for the Democratic-led House to approve articles of impeachment – formal charges – against Trump. That would lead to a trial in the Senate on whether to convict Trump and remove him from office. Republicans control the Senate and have shown little support for Trump’s removal.

SONDLAND’S TESTIMONY HIGHLY ANTICIPATED

Democrats are looking into whether Trump abused his power by withholding $391 million in aid to Ukraine as leverage to get Kiev to investigate Biden, who is a leading contender for the Democratic nomination to take on Trump in 2020. The money, approved by the US Congress to help US ally Ukraine combat Russia-backed separatists, was later provided to it.

Most highly anticipated, however, will be Sondland, both because of the central role he has played as well as other witnesses’ statements about his dealings with Trump on Ukraine.

David Holmes, a US embassy official in Kiev, told lawmakers in closed-door testimony, that he overheard a phone call between Trump and Sondland, a former political donor, in which the ambassador told the president his Ukrainian counterpart was ready to carry out the investigations. The phone call occurred on July 26, one day after the phone conversation between Trump and Zelenskiy.

The testimony by Holmes ties Trump more directly to the effort to pressure Ukraine. Holmes’ statement also appears to contradict Sondland’s previous sworn testimony, which he has already revised once, about his interactions with Trump.

Democratic Senator Chris Murphy told CNN on Sunday that Sondland “has to decide whether his primary loyalty is to America or to the president of the United States.”

Republican Congressman Jim Jordan, a member of the committee and staunch Trump defender, told CBS “Face the Nation” about Sondland: “What I also know is he said there was never any quid pro quo (sought by Trump, according to Sondland’s text message with the acting U.S. ambassador to Ukraine) … So, we’ll have him in front of us and we’ll find out.”