Treasury Secretary Steven Mnuchin has refused to hand over Trump's returns,maintaining that the Democratic-controlled House is seeking them for political purposes. The Democrats say they're needed for legitimate congressional oversight.

The demand for documents ended a tumultuous week that featured an increasingly intense battle between the White House and Congress.

Late on Friday,Trump ordered a substantial reduction in the staff of the National Security Council,according to five people familiar with the plans.

Some of the people described the staff cuts as part of a White House effort to make its foreign policy arm leaner under new National Security Adviser Robert O'Brien.

The request to limit the size of the NSC staff was conveyed to senior agency officials by acting White House Chief of Staff Mick Mulvaney and O'Brien this week.

Earlier on Friday,local time,The New York Times reported that a second intelligence official with concerns and more direct knowledge regarding President Donald Trump's dealings with Ukraine was considering filing a whistleblower complaint.

Trump said he would formally object to Congress about the House impeachment inquiry,even as he acknowledged that Democrats"have the votes"to proceed.

"I really believe that they're going to pay a tremendous price at the polls,"Trump said.

But Democrats accused Trump of speeding down"a path of defiance,obstruction and cover-up"and warned that defying the House subpoena would in itself be considered"evidence of obstruction"that could be included among articles of impeachment.

Democrats have made Trump's request in July that Ukraine investigate former Vice President Joe Biden the centrepiece of their probe. A whistleblower complaint said that Trump sought to use military assistance for Ukraine as leverage to push President Volodymyr Zelensky to investigate the 2020 Democratic hopeful.

"We deeply regret that President Trump has put us - and the nation - in this position,but his actions have left us with no choice,"wrote the three Democratic House chairmen,Representative Elijah Cummings,Adam Schiff and Eliot Engel,in issuing Friday's subpoena after White House resistance to the panel's request for witnesses and documents.

White House press secretary Stephanie Grisham derided the subpoena as coming from a Democratic"kangaroo court."

But Pelosi insisted the House is well within its rules to conduct oversight of the executive branch under the US Constitution.

In the letter accompanying the subpoena,the three chairmen agreed,stating,"Speaker Pelosi has confirmed that an impeachment inquiry is underway,and it is not for the White House to say otherwise."

Trump's comments at the White House came shortly before Democrats sent a separate extensive request for documents to Vice President Mike Pence about his contacts with Ukraine.

Pence spokeswoman Katie Waldman dismissed the demand,saying that given its wide scope,"it does not appear to be a serious request."

The House has also subpoenaed Secretary of State Mike Pompeo.

Late Thursday,Houseinvestigators released a cache of text messages that showed top US diplomats encouraging Ukraine's newly elected president to conduct an investigation linked to Biden's family in return for granting a high-profile visit with Trump in Washington.

The release followed a 10-hour interview with one of the diplomats,Kurt Volker,who stepped down as special envoy to Ukraine after the impeachment inquiry had begun.

On Friday,investigators in Congress heard again from Michael Atkinson,the intelligence community inspector general who brought forward the whistleblower complaint of Trump's call with the Ukraine president that sparked the impeachment inquiry.

Trump repeated on Friday that he had been pressing Ukraine to investigate corruption,not trying to undermine Biden,who could be his 2020 presidential election opponent. He made a related request of China,specifying Biden and his son,on Thursday.

As Republicans search for a response to the investigation,the absence of a procedural vote to begin the probe has been a main attack line against Democrats.

Pelosi swatted back the need for such a vote as unnecessary.

"The existing rules of the House provide House Committees with full authority to conduct investigations for all matters under their jurisdiction,including impeachment investigations,"Pelosi wrote Thursday in a letter to House GOP Leader Kevin McCarthy after he,too,pressed for a floor vote.

There's no clear-cut procedure in the Constitution for initiating an impeachment inquiry,leaving many questions about possible presidential obstruction untested in court,said Allan Lichtman,a history professor at American University.

"There's no specification in the Constitution in what does and does not constitute a more formal impeachment inquiry or investigation,"he said.

with AP,Bloomberg