President Donald Trump's personal attorney, Rudy Giuliani, is defying a subpoena from House lawmakers investigating impeaching the president.

Giuliani told ABC News that "if they enforce [the subpoena], then we will see what happens."

He also said he had parted ways with his own attorney, Jon Sale.

Giuliani is being probed by lawmakers over his role in seeking to pressure Ukraine to open an investigation into Joe Biden, a key domestic rival of President Trump.

Federal prosecutors in New York are investigating Giuliani over possible lobbying law violations and over his business dealings in Ukraine.

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President Donald Trump's personal attorney, Rudy Giuliani, is defying a House subpoena from Democrats probing possible impeachable offences by US President Donald Trump.

Giuliani told ABC News Tuesday that "if they enforce [the subpoena], then we will see what happens."

He also said that he was no longer retaining the services of Jon Sale, an attorney he hired earlier in October.

Democrats requested documents from Giuliani as part of their impeachment inquiry into a whistleblower's concerns that President Trump sought to improperly influence Ukraine to investigate his domestic political rival, Democratic presidential contender Joe Biden.

In a letter to House committees investigating impeachment, Sale laid out the reasons why Giuliani would not cooperate with the subpoena request.

His defense echoes that of the Trump administration, which has defied impeachment probe congressional subpoenas. It says that the House investigation is illegitimate because it has not formally voted to launch an impeachment inquiry.

Giuliani is also reportedly under investigation by federal prosecutors in the office he once led as a prosecutor in New York, who are probing his business dealings in Ukraine and whether he violated foreign lobbying laws.

The former New York City mayor played a key role in lobbying for Ukrainian officials to open an investigation into Biden and his son, Hunter.

Giuliani was pursuing allegations that have not been substantiated that the former vice president sought to quash a probe into an energy firm where his son worked.

Last week two associates of Giuliani, Lev Parnas and Igor Fruman, were arrested with one-way-tickets to Europe on charges on election law violations.

The two men have not yet entered a plea, reported USA Today.