Hearings in the Democrat-controlled House start Wednesday at 10 a.m. ET (1500 GMT).
Trump, a Republican, and his lawyers were invited earlier to appear, but declined on Sunday, citing a lack of “fundamental fairness.”
Here is who will testify:
Noah Feldman, a professor of law at Harvard Law School. Feldman, a former clerk for Justice David Souter of the US Supreme Court, was senior constitutional adviser to the Coalition Provisional Authority in Iraq.
Feldman, in opinion columns for Bloomberg News, has written https://www.bloomberg.com/opinion/articles/2019-11-11/in-trump-impeachment-hearings-democrats-must-avoid-legalism that Democrats have legitimate grounds to move ahead with impeachment because Trump has abused his power in office.
Pamela Karlan is a professor of public interest law at Stanford Law School, oversaw voting rights at the Justice Department under former President Barack Obama and served as a law clerk to Associate Justice Harry Blackmun at the US Supreme Court.
Karlan in October argued before the Supreme Court that US employees should not be fired because of their sexual orientation. She has not been a vocal public commentator on Trump's impeachment proceedings. Michael Gerhardt, the professor of jurisprudence at the University of North Carolina School of Law, is a former media director for Al Gore's presidential campaign, and served as a joint witness during the impeachment proceeding of Democratic former President Bill Clinton.
He has written a book called "Impeachment: What Everyone Needs to Know," that claims to be a non-partisan "primer for anyone eager to learn about impeachment’s origins, practices, limitations, and alternatives." Jonathan Turley, a professor of public interest law at the George Washington University Law School, has written opinion columns arguing that Democrats have moved too quickly, focused too narrowly on Trump's relations with Ukraine, and suffered from the lack of testimony from people who were directly involved, like Rudy Giuliani.
Turley testified as a constitutional expert https://jonathanturley.org/2007/08/20/clinton-impeachment-testimony-house-judiciary-committee in the Clinton impeachment, telling the House Judiciary Committee in 1998 that the allegations against Clinton, which included perjury, obstruction of justice, witness tampering and abuse of office, would amount to "clear and compelling" grounds for impeachment if proven.
Turley also said this spring that Trump's efforts to interfere with a Justice Department investigation of Russia's involvement in the 2016 election, as outlined in Special Counsel Robert Mueller's report, did not amount to obstruction of justice.