The witnesses testifying in public impeachment hearings

Washington — At the heart of the impeachment inquiry is whether President Trump withheld U.S. aid to Ukraine and a White House meeting in exchange for Ukraine announcing investigations that would help him politically. Democrats allege the president orchestrated an "extortion scheme."

The Democratic-controlled House launched an impeachment inquiry after a whistleblower filed a complaint about a July 25 phone call between Mr. Trump and Ukraine's president, Volodymyr Zelensky. But government officials had sounded similar alarms weeks before that.

In June, Mr. Trump told ABC News that he would listen — and not necessarily call the FBI, as instructed — if a foreign country had damaging information on his political rivals. He called it opposition research and disagreed that it would amount to election interference.

Those comments prompted the chair of the Federal Elections Commission to tweet a public reminder that "it is illegal to solicit, accept, or receive anything of value from a foreign national in connection with a U.S. election."

"Electoral intervention from foreign governments has been considered unacceptable since the beginnings of our nation," wrote FEC Chair Ellen Weintraub.

Fast forward to this week, when the public will hear from three of the 15 current or former White House officials who have testified behind closed doors. The transcripts of their testimonies were released last week. Here's what we learned from them (and how to watch this week's hearings):

Bill Taylor (Testifying on Wednesday)

Taylor says aide overheard Trump ask Sondland about "the investigations" one day after Ukraine call

Taylor: There were "regular" and "highly irregular" Ukraine policy channels

​George Kent: Giuliani's efforts were "infecting" Ukraine policy