Former US National Security Adviser John Bolton. Photo: Reuters / Joshua Roberts.

Democratic lawmakers have summoned several little-known US officials to testify this week in their impeachment inquiry of President Donald Trump but have their eye on a more prominent player — former National Security Adviser John Bolton.

The House of Representatives Democrats running the inquiry want to hear first-hand from the mustachioed foreign-policy hawk who, according to the testimony of other officials, was alarmed by a White House effort to pressure the president of Ukraine to investigate Trump’s political rivals.

“Obviously he has very relevant information and we want him to testify,” House Intelligence Committee Chairman Adam Schiff, one of the Democrats leading the effort, told ABC‘s “This Week” on Sunday, referring to Bolton.

That may not be easy. White House officials have tried, with some success, to block current and former government officials from testifying in the inquiry. Secretary of State Mike Pompeo, acting White House Chief of Staff Mick Mulvaney and Trump’s personal lawyer, Rudy Giuliani, are among those who have defied House subpoenas for documents, although other witnesses have appeared before the committee despite orders from the administration to steer clear.

The White House says the probe is illegitimate because the House did not hold a formal vote in order to start it. A federal court said on Friday that no vote was needed.

Congressional sources told Reuters last week that they may call Bolton to testify in public hearings that are expected to start in the coming weeks. On Sunday, Schiff said he anticipated the White House would try to bar the former aide from appearing, but did not say whether he would he planned to call Bolton to testify in public or behind closed doors.

Bolton, who was fired by Trump in September after the pair clashed over how to handle foreign policy challenges, has not commented publicly on the impeachment effort.

Roadblocks

Amid the legal roadblocks thrown up by the White House, Monday’s scheduled witness, former deputy national security adviser Charles Kupperman, could fail to show up. Kupperman’s lawyer says lawmakers should wait for a court to rule whether he should comply with a congressional subpoena or honor the Trump administration’s order not to testify.

The Democratic-led House committees are focusing on Trump’s July request to Ukrainian President Volodymyr Zelenskiy that he investigate former US Vice President Joe Biden, a leading 2020 Democratic presidential candidate, and his son Hunter Biden, who had served on the board of a Ukrainian gas company.

US law prohibits candidates from accepting foreign help in an election.

Acting US Ambassador to Ukraine William Taylor testified last week that Trump withheld $391 million in security assistance to Ukraine in an effort to get Zelenskiy to publicly commit to investigate both the Bidens and a debunked conspiracy theory that Ukraine — not Russia — meddled in the US 2016 presidential election.

Lawmakers also hope to hear from Alexander Vindman, the White House National Security Council’s top Ukraine expert, on Tuesday. On Wednesday, they have lined up Kathryn Wheelbarger, the acting assistant secretary of defense for international security, and two State Department Ukraine specialists — Catherine Croft and Christopher Anderson.

Tim Morrison, a top White House adviser on Russia and Europe, is scheduled for Thursday.