Watch: Four law professors testify on Day 1 of House Judiciary impeachment hearings

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Four law professors take the stand Wednesday as the House Judiciary Committee begins public impeachment hearings into President Donald Trump.

Testifying are Noah Feldman of Harvard Law School, Pamela Karlan of Stanford Law School, Michael Gerhardt of the University of North Carolina School of Law and Jonathan Turley of the George Washington University Law School.

The hearing follows two weeks of explosive public testimony before the House Intelligence Committee. Democrats are using the hearings to determine whether Trump abused his power by withholding necessary aid from Ukraine while pressuring its president, Volodymyr Zelenskiy, to investigate Democratic presidential contender Joe Biden.

In a party-line vote, the House Intelligence Committee on Tuesday approved its impeachment report that accuses Trump of obstructing the inquiry and leveraging a White House meeting and military aid to Ukraine in exchange for an investigation into Biden and other Trump rivals.

House Judiciary Committee Chairman Jerry Nadler invited Trump or his representatives to attend Wednesday's hearing, but the president declined, accusing House Democrats of scheduling the hearing while he's in London for the NATO summit.

In a statement, Nadler acknowledged Trump's refusal and called it "unfortunate because allowing the President to participate has been a priority for the House from the outset."

"The American people deserve transparency," the New York Democrat continued. Referring to Trump's infamous July 25 phone call with Zelenskiy, Nadler added: "If the President thinks the call was 'perfect' and there is nothing to hide then he would turn over the thousands of pages of documents requested by Congress, allow witnesses to testify instead of blocking testimony with baseless privilege claims, and provide any exculpatory information that refutes the overwhelming evidence of his abuse of power."

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