The third day of televised hearings in the House impeachment inquiry into President Donald Trump is set to begin at 9 a.m. ET Tuesday. The YouTube livestream is available here:

WATCH LIVE: Open Hearing with Lt. Col. Alexander Vindman and Jennifer Williams youtu.be

Who is testifying?

Lt. Col. Alexander Vindman, an Army officer at the National Security Council, and Jennifer Williams, his counterpart at Vice President Mike Pence's office, will testify before the House Intelligence Committee at 9 a.m.

Later in the afternoon, at 2:30 p.m., NSC official Timothy Morrison and former Ukraine special envoy Kurt Volker will testify before the House.

What is the background?

Vindman and Williams, the morning witnesses, have expressed concern over President Trump's July 25 phone call with Ukrainian President Volodomyr Zelensky, of which they were direct witnesses who listened in on the call. Democratic lawmakers will undoubtedly press them on why they felt Trump calling for an investigation into former Vice President Joe Biden and his son, Hunter Biden, was inappropriate. They will also be asked to provide more detail on any quid pro quo withholding of military aid by the president.

Both Vindman and Williams, being that they are White House aides, also bring the inquiry closer to Trump than ever before. Last week's hearings with testimony from career diplomats Bill Taylor and George Kent, along with ex-Ambassador to Ukraine Marie Yovanovitch, were charged with dealing in mostly hearsay and secondary source material.

The afternoon witnesses, Morrison and Volker, were requested by Republicans, who have argued that their testimony will undercut Democrats' claims of any wrongdoing by the President.

Democrats hope to find holes in their stories and squeeze their testimonies to see if their minds have changed since hearing from other witnesses since they testified at closed-door meetings several weeks ago.

According to a report from the New York Times Tuesday morning, Volker may modify his testimony, now saying he didn't know that military aid was tied to the call for investigations, or essentially, that he was kept out of the loop.