Donald Trump, only fourth US president to face impeachment enquiry

For only the fourth time in US history, the House of Representatives has started a presidential impeachment inquiry.

House committees are trying to determine if President Donald Trump violated his oath of office by asking Ukraine to investigate political rival Joe Biden and his family, and to investigate the country's involvement in the 2016 US presidential election.

The three House committees leading the Democratic investigation have scheduled several current and former National Security Council officials to testify this week behind closed doors — an attempt to get a better look inside the White House as Trump pushed Ukraine to conduct politically motivated investigations.

The officials include Charles Kupperman, a former deputy at the NSC under then-national security adviser John Bolton, and current NSC staffers Tim Morrison and Alexander Vindman. Morrison is particularly significant. William Taylor, the top US diplomat in Ukraine, told lawmakers in his deposition last week about phone calls he had with Morrison that described the Ukraine effort.

The chairman of the House Intelligence Committee, Rep. Adam Schiff, who is leading the inquiry, said Sunday he would like Bolton to testify but expects the White House would "fight us" over his appearance. Bolton is "a very important witness" who has "very relevant information," Schiff, D-Calif., told ABC's "This Week." The committees are scheduled to hear from three other State Department and Defense Department witnesses as well. Lawmakers want to determine whether military aid to Ukraine was held up as a condition of the investigations.

The Democrats are moving quickly, sometimes scheduling multiple depositions in one day. They're trying to compile facts and eventually transition to public hearings. Schiff said Saturday that the committees are making "rapid progress." He told ABC that "we will be doing public hearings, and I think we'll be doing them soon." It is unclear if all of the officials will appear because Trump has pledged to obstruct the probe. So far, most witnesses have decided to testify after receiving subpoenas from the committees.

One of the witnesses, Kupperman, has taken the extraordinary step of asking a federal court who he should listen to — Congress or Trump.

After he was subpoenaed, Kupperman filed a lawsuit in federal court on Friday asking a court whether he should accede to House demands for his testimony or to assert "immunity from congressional process" as directed by Trump.

In the lawsuit, Kupperman said he "cannot satisfy the competing demands of both the legislative and executive branches." Without the court's help, he said, he would have to make the decision himself — one that could "inflict grave constitutional injury" on either Congress or the presidency.

A judge on Friday ordered the Justice Department to give the House secret grand jury testimony from special counsel Robert Mueller's Russia investigation , handing a victory to Democrats who want the material for the impeachment inquiry.

In ordering the department to turn over the material by October 30, Chief US District Judge Beryl Howell affirmed the legality of the impeachment inquiry itself. The Mueller materials could reveal previously hidden details to lawmakers about Trump's actions during the 2016 election and become part of the impeachment push.

The Trump administration could appeal the decision, however, further delaying the release of the materials.