US House Democrats are moving aggressively against Trump lawyer Rudy Giuliani as part of their impeachment inquiry.

In this file photo taken on November 17, 2016 Former New York City mayor Rudy Giuliani arrives at the Trump Tower for meetings with US President-elect Donald Trump, in New York. (Photo: AP)

Giuliani has been at the heart of President Donald Trump's efforts to get Ukraine to investigate political rival Joe Biden's family.

On Monday they issued a subpoena to the former New York mayor for text messages, phone records and other communications that they referred to as possible evidence. They also are seeking documents and depositions from three of Giuliani's business associates.

The circle of officials with knowledge of Trump's phone call to Ukraine's president widened with the revelation that a Cabinet official, Secretary of State Mike Pompeo, listened in on the July 25 conversation. That call and the circumstances surrounding it are fueling the new Democratic drive for impeachment.