Next up in Donald Trump impeachment hearing: A parade of key witnesses

Several more witnesses scheduled to testify in the House impeachment hearings over the next week are expected to say they too worried about President Donald Trump's push for Ukraine to investigate Democrats as the US withheld military aid from the country.

The House intelligence committee, which is conducting the impeachment hearings, has set a packed schedule of open hearings over the next week.

On Friday, lawmakers will hear from former US Ambassador to Ukraine Marie Yovanovitch, who was ousted in May at Trump's direction.

She told lawmakers in a closed-door deposition last month that there was a "concerted campaign" against her as Trump and his personal lawyer Rudy Giuliani pushed for probes of Democrat Joe Biden and other political opponents.

Eight more witnesses will testify next week, some in back-to-back hearings on the same day. Among them will be Lt. Col. Alexander Vindman, a National Security Council official who said he raised concerns in the White House about Trump's push for investigations; Gordon Sondland, Trump's European Union ambassador, who spoke to the president about the Ukraine policy; and Fiona Hill, a former Russia adviser to the White House who told lawmakers about national security adviser John Bolton's concerns about Ukraine.

All witnesses testifying this week and next have already spoken to investigators in closed depositions, some of them for 10 hours or more.

Though those private depositions are largely done, Democrats have scheduled two more for this week at the same time they are conducting the open hearings.

Democrats have scheduled a closed-door session with David Holmes, the political counsellor at the US Embassy in Kyiv, for Friday.

An official familiar with the matter said Holmes is the person Taylor referred to in his testimony on Wednesday when he said an aide had overheard a conversation between Sondland and Trump in July about Ukraine conducting investigations.

They have also scheduled a Saturday deposition with Mark Sandy, an official at the Office of Management and Budget. Sandy is one of several OMB officials who have been invited by the committee to appear as lawmakers try to find out more about the military aid that was withheld.

So far, none of those officials has shown up for their depositions as Trump has instructed his administration not to cooperate.

While the open hearings are being conducted by the intelligence panel, the closed-door hearings have been held by the intelligence, Foreign Affairs and Oversight and Reform committees.

The public hearings are expected to last at least another week. After that, the three committees will submit a report to the Judiciary panel, which will oversee the impeachment process.

Judiciary is expected to hold its own hearings and, eventually, vote on articles of impeachment. Democrats say they are still deciding whether to write them.

Next would come a floor vote, and if articles of impeachment are approved by the House, there would then be a Senate trial.

House Democrats are hoping to finish the process by the end of the year. A Senate trial, if called for, would likely come in 2020.