Since Speaker Nancy Pelosi announced a formal impeachment inquiry in September, House investigators have conducted weeks of fact-finding largely in private to determine if President Trump abused the power of his office in his dealings with Ukraine. On Wednesday, the inquiry enters a new phase: public hearings.

The Basics

Who: William B. Taylor Jr., the top American diplomat in Ukraine, and George P. Kent, a senior State Department official in charge of Ukraine policy, will appear for joint testimony.

What: The House Intelligence Committee, led by its chairman, Representative Adam B. Schiff of California, will offer lawmakers from both parties a chance to question the witnesses.

When and Where: The proceedings start at 10 a.m. Eastern in the one of the grandest hearing rooms the House has: the vaulted, columned chambers of the Ways and Means Committee. (Democrats intentionally chose the chambers as the backdrop.) We expect the hearing to go until midafternoon.

How to Watch: The New York Times will stream the testimony live, and a team of reporters in Washington will provide real-time context and analysis of the events on Capitol Hill. Follow along at nytimes.com, starting a few minutes before 10 a.m.

Four key issues will dominate the hearings.

The Democratic case against the president is built around four major issues that the president’s critics say prove that Mr. Trump abused his office:

Giuliani and a shadow diplomacy

Current and former diplomats and national security officials have described to investigators how Rudolph W. Giuliani, the president’s personal lawyer, led a shadow foreign policy in Ukraine that circumvented career foreign service officers and established diplomatic channels in an effort to further Mr. Trump’s political agenda.

Mr. Kent told investigators that Mr. Giuliani’s influence over Mr. Trump when it came to Ukraine was “almost unmissable,” and that Mr. Giuliani led a “highly irregular” channel of policymaking on Ukraine.

The security aid

Congress approved nearly $400 million in security aid to help Ukraine counter Russian aggression. But witnesses have testified that the aid was abruptly frozen at the direction of the acting chief of staff, Mick Mulvaney, and the Office of Management and Budget. They have said they were alarmed to learn that the aid would not be restarted unless Ukraine agreed to publicly announce the investigations that Mr. Trump wanted.

In gripping closed-door testimony, Mr. Taylor recalled having seen firsthand Ukraine’s need for the security aid when he traveled to the country and witnessed its soldiers facing Russian forces across a damaged bridge. He told lawmakers that he knew at that time that “more Ukrainians would undoubtedly die without the U.S. assistance.”

The July 25 call

At the heart of the Democratic case is the reconstructed transcript released by the White House of the July phone call between Mr. Trump and Ukraine’s president, Volodymyr Zelensky. In the call, Mr. Trump asked Mr. Zelensky, newly elected as Ukraine’s leader, to “do us a favor” and investigate the former vice president, as well as a conspiracy theory that asserts that Ukraine, not Russia, interfered in the 2016 election to help Democrats, not Mr. Trump.

Both sides have used the call to bolster their arguments. Democrats say it is direct evidence of Mr. Trump’s culpability in the pressure campaign. Republicans note that there is no direct mention during the call of the security aid and argue it falls far short of providing proof that the president was involved in abusing his office. Mr. Trump has insisted repeatedly that the call was “perfect” and exonerates him.

The ouster of Marie L. Yovanovitch

Among the most personal moments during Wednesday’s hearing is likely to be discussion of the ouster of Ms. Yovanovitch from her post as ambassador to Ukraine. Several witnesses have testified about a smear campaign led by Mr. Giuliani to damage Ms. Yovanovitch’s reputation and oust her.

Philip T. Reeker, the acting assistant secretary in charge of European and Eurasian Affairs, told investigators that the effort to cast Ms. Yovanovitch as a “liberal outpost” was a “fake narrative” that “really is without merit or validation.”
— Michael D. Shear