As House Democrats move forward with their impeachment inquiry against President Donald Trump, they have started issuing subpoenas to senior administration officials and Trump associates involved in the president's dealings with Ukraine.

According to Reuters, a subpoena is "a legally enforceable demand for documents, data, or witness testimony."

Congress has broad power to issue subpoenas, but the Supreme Court has ruled it must further "some legitimate legislative purpose."

Here are the Trump associates who have been subpoenaed by House Democrats in the Trump impeachment inquiry.

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As House Democrats move forward with their impeachment inquiry against President Donald Trump, they have started issuing subpoenas to senior administration officials and Trump associates involved in the president's dealings with Ukraine.

Secretary of State Mike Pompeo and Trump's personal lawyer, Rudy Giuliani, were both subpoenaed in the investigation.

Trump faces an impeachment inquiry over a July 25 phone call with Ukrainian President Volodymyr Zelensky. During that call, Trump pressured Zelensky to probe former vice president Joe Biden and his son, Hunter Biden, on unsubstantiated allegations of corruption. The reports spurred House Speaker Nancy Pelosi to launch impeachment proceedings against Trump — and House Democrats are trying to move forward with a speedy, narrow investigation.

Read more: Here are all the documents that lay out the allegations in the Trump-Ukraine scandal

According to Reuters, a subpoena is "a legally enforceable demand for documents, data, or witness testimony." Congress has broad power to issue subpoenas, but the Supreme Court has ruled it must further "some legitimate legislative purpose."

Though House Democrats have investigated Trump's family members, key policy decisions and other White House practices since assuming the majority in early 2019, the White House has stonewalled many of the probes. Some legal experts have argued that impeachment proceedings strengthen the ability of Congress to compel the executive branch to turn over information and produce witnesses.

Here are the Trump associates who have been subpoenaed by House Democrats in the Trump impeachment inquiry.