Since the start of the impeachment inquiry six weeks ago, more than a dozen current and former Trump administration officials have refused to testify before House of Representatives investigators, raising questions about Congress' ability to summon key witnesses.
In the latest instance, acting White House Chief of Staff Mick Mulvaney failed to show up for a scheduled deposition on Friday, despite a subpoena issued by the House Intelligence Committee.
Lawmakers' strongest investigative tool is the subpoena — a legal order to appear before a congressional committee. But Congress has had mixed success over the years in utilizing this mechanism to compel testimony.
While Mulvaney, a former Republican House member, is unlikely to cooperate, more than a dozen other officials have stepped forward, in many cases after being subpoenaed.
With the testimony of these officials from the White House, the State Department and the Pentagon and other evidence, House Democrats appear confident they have enough to build a case that Trump abused his power when he pressed the president of Ukraine over the summer to investigate Trump's political rivals while military aid to Ukraine was withheld.
Here are four things you need to know about congressional subpoenas:
What is a congressional subpoena?
A congressional subpoena is similar to a grand jury subpoena, a legal order issued to a recalcitrant witness to produce testimony and documents in connection with an investigation. Witnesses — private citizens and government officials alike — are typically requested to provide information on a voluntary basis. When they refuse to do so, congressional committees can serve them with subpoenas to compel their compliance.
What is the source of Congress' subpoena power?
While there are no constitutional provisions that explicitly give Congress the authority to investigate the executive branch and issue subpoenas, the Supreme Court has interpreted the Constitution to imply a power to conduct such investigations, according to Kimberly Wehle, a law professor at the University of Baltimore and author of "How to Read the Constitution."
"It's implied in its power to make laws and its power to impeach," Wehle said of Congress' power to investigate. "It has to find facts in order to legislate and decide whether to take impeachment action."
As part of that broad authority, congressional committees can first ask witnesses to testify and produce documents and then subpoena them if they refuse to cooperate.
Can subpoenas be ignored?
Every recipient of a congressional subpoena has a legal obligation to comply. "There is no blanket immunity from having to show up," Wehle said.
However, while private citizens can find it hard to defy a congressional subpoena, administration officials unwilling to testify possess an oft-used evasive tool: executive privilege.
"That is a constitutionally recognized doctrine which basically safeguards the communications of the president and senior officials and others that work in the executive branch," said Hans von Spakovsky, a senior legal fellow at the Heritage Foundation.
Nearly as many officials have simply ignored subpoenas in the impeachment inquiry as have complied, while a former deputy national security adviser, Charles Kupperman, has asked a federal judge to rule on whom he should obey: the White House or Congress.
"A private citizen cannot sue Congress and try to avoid coming in when they're served with a lawful subpoena," Representative Adam Schiff, chairman of the House Intelligence Committee, said after Kupperman failed to show up for his scheduled deposition on Oct. 28.
How does Congress enforce subpoenas? Are there penalties for noncompliance?
Congress has a couple of mechanisms to enforce subpoenas issued to executive branch officials.It can petition a federal court and try to convince a judge that the executive branch official is legally obligated to comply. Alternatively, it can ask the Justice Department to bring contempt-of-Congress charges against the defiant party, although Democratic investigators likely would get a cool reception from Attorney General William Barr, a strong proponent of executive power.
In theory, there is a third way for lawmakers to gain compliance: sending the House sergeant at arms to arrest anyone who refuses to comply. But that's an option that hasn't been in use since the early days of the American republic.
Members of Congress have floated various ideas about how to strengthen compliance over the years, including requiring courts to expedite subpoena enforcement lawsuits brought by Congress.
But, said von Spakovsky, Congress is unlikely to go down that route.
"This boils down to a basic constitutional fight, a political fight that involves the separation of powers between the congressional branch and the executive branch. Sometimes the executive branch wins. Sometimes the Congress wins," he said.