CAPITOL HILL - The U.S. House of Representatives holds its first public hearings this week on the impeachment inquiry into President Donald Trump with testimony by three top diplomats on Wednesday and Friday.

William Taylor, the highest-ranking U.S. diplomat in Ukraine, and George Kent, a senior State Department official in charge of U.S. policy toward Ukraine, are set to testify on Wednesday, followed by Marie Yovanovitch, the ousted former U.S. ambassador to Ukraine, on Friday.

All three diplomats have previously testified behind closed doors about Trump's efforts to pressure Ukraine to investigate former Vice President Joe Biden, and probe a discredited conspiracy theory regarding the 2016 president election.

Democrats say the open hearings will allow the public to assess the credibility of the witnesses and their testimonies. Republicans are likely to attempt to discredit the impeachment proceedings and poke holes in the witnesses' testimony.

Here is what you need to know about the three witnesses and their role in the Ukraine affair.

William Taylor

Taylor has served as charge d'affaires of the U.S. Embassy in Kyiv since June after Trump abruptly recalled Yovanovich. Secretary of State Mike Pompeo asked Taylor to step into her role. A West Point and Harvard-educated former Army officer and career diplomat, Taylor previously served as U.S. ambassador to Ukraine from 2006 to 2009. In what Democratic House member Debbie Wasserman Schultz of Florida called "the most damning testimony I've heard," Taylor told House investigators last month that Trump had explicitly demanded that Ukraine investigate Biden, his son Hunter and other Democrats in exchange for releasing U.S. military aid. The testimony, based on Taylor's conversation with Gordon Sondland, the U.S. envoy to the European Union and a Trump campaign donor, contradicted Trump's assertion that there was no "quid pro quo" with Ukraine. The White House dismissed the testimony as hearsay. Taylor's text messages to Sondland, in which he said it was "crazy" the administration was freezing Ukraine aid for political investigations, are among the impeachment evidence against Trump.

George Kent

As the deputy assistant secretary of state for European and Eurasian affairs, Kent oversees U.S. policy toward Ukraine, Moldova, Belarus, Georgia, Armenia and Azerbaijan. Like Taylor, Kent, a 27-year veteran of the foreign service, was sidelined by what he described as "unusual channels" of diplomacy toward Ukraine run by Trump lawyer Rudy Giuliani, Energy Secretary Rick Perry, Sondland and Kurt Volker, the former U.S. special representative for Ukraine. In his closed-door testimony last month, Kent told investigators that Giuliani had been pressing the Ukrainians to conduct "politically motivated prosecutions that were injurious to the rule of law, both in Ukraine and the U.S. He also testified that Sondland "had talked to the president ... and POTUS wanted nothing less than (Ukrainian) President (Volodymyr) Zelenskiy to go to a microphone and say investigations, Biden and Clinton."

Marie Yovanovitch

A career diplomat, Yovanovitch was U.S. ambassador to Ukraine from July 2016 to May 2019, when she was unceremoniously recalled to Washington after Giuliani and his allies waged what her colleagues and Democrats have described as a smear campaign against her. Two Giuliani associates recently arrested for campaign finance violations are accused of lobbying former Republican House member Pete Sessions of Texas for her ouster. Her removal sent shockwaves through the foreign service, with more than 50 former female U.S. ambassadors writing a letter to Trump and Pompeo to protect foreign service officers from political retaliation. Yovanovitch testified last month that she felt threatened, and worried about her safety after Trump said "she's going to go through some things." She also told lawmakers that Sondland had recommended she praise Trump on Twitter if she wanted to save her job.