Washington, Oct 6 (AP) Secretary of State Mike Pompeo said the State Department intends to "follow the law" in the House impeachment investigation and vigorously defended President Donald Trump, dismissing questions about the president's attempts to push Ukraine and China to investigate a Democratic political rival.

The Trump administration and House Democrats often disagree about what the law requires, leaving open the question of how Pompeo may interpret Democrats' demands for key information about Trump's handling of Ukraine.

Pompeo, speaking in Greece, said the State Department sent a letter to Congress Friday night as its initial response to the document request and added, "We'll obviously do all the things that we're required to do by law."

He has allowed Democrats to interview a series of witnesses next week. The administration has struggled to come up with a unified response to the quickly progressing investigation.

Democrats have warned that defying their demands will in itself be considered "evidence of obstruction" and a potentially impeachable offense.

Pompeo has become a key figure in the Democrats' investigation.

He was on the line during the July phone call in which Trump pressed Ukrainian President Volodymyr Zelenskiy to investigate former Vice President Joe Biden and his son, Hunter — sparking a whistleblower complaint and now the impeachment inquiry.

Pompeo had initially tried to delay a handful of current and former officials from cooperating with the inquiry and accused Democrats trying to "bully" his staffers. On Saturday, Pompeo did not back off his defense of Trump's call with Ukraine.

"There has been some suggestion somehow that it would be inappropriate for the United States government to engage in that activity and I see it just precisely the opposite," he said.

Trump has offered a series of contradictory statements when it comes to the Democrats' subpoena of White House records.

Asked on Wednesday whether the White House intended to comply, Trump told reporters, "I always cooperate," even as he dismissed the inquiry as "a hoax."

A day later, however, Trump had a different answer for the same question, saying he would instead leave the matter to his lawyers. "That's up to them to decide," he said, "But the whole investigation is crumbling."

By Friday, however, Trump confirmed reports the White House was preparing a letter to House Speaker Nancy Pelosi, D-Calif., arguing that Congress cannot undertake an impeachment investigation without first having a vote to authorize it.

Pelosi has insisted the House is well within its rules to conduct oversight of the executive branch under the Constitution regardless. It was unclear on Saturday when or if that letter would be sent.

Pompeo, meanwhile, made clear that the State Department had yet to turn over any document, but intended to follow a proper review.

And he said he would do so faster than the Obama administration.

"I remember precisely once when I was on that side and we were looking for documents, I remember precisely how long it took for those documents to come across," he said in an apparent reference to his experience as a congressman during the investigation into the 2012 attack on the US mission in Benghazi, Libya.

"We're going to beat that. We're going to be more responsive than the Obama administration was in the years that preceded this particular Congress," he said.

A congressional aide familiar with Pompeo's response confirmed that the State Department had indeed been in contact, even if Pompeo had failed to meet a Friday deadline to produce documents required by the subpoena.

Trump, meanwhile, continued to seethe Saturday, denouncing the investigation as yet another "Witch Hunt!" and "a fraud against the American people!"

In a series of tweets Saturday, including several sent as his presidential motorcade ferried him back and forth to his Virginia golf course, Trump defended his conduct and lashed out at critics, including a past foil, Senator Mitt Romney, R-Utah.

"I'm hearing that the Great People of Utah are considering their vote for their Pompous Senator, Mitt Romney, to be a big mistake. I agree! He is a fool who is playing right into the hands of the Do Nothing Democrats!" Trump wrote.

He ended his tweet with an extraordinary call to impeach a senator from his own party.

Yet Romney was joined Saturday in his criticism by a second Republican senator, Susan Collins of Maine, who said "it's not OK" for a president to encourage a foreign state to investigate a political rival.

Speaking after a firefighter memorial service in Maine, Collins said Trump made a "big mistake" when he piled on his outreach to Ukraine by publicly calling on China to investigate the Bidens.

Senator Ben Sasse of Nebraska is the only other Republican senator to publicly criticize the president's comments that further fueled an impeachment inquiry.

Lawmakers are focused on Trump's efforts to push Ukraine to investigate the Bidens.

A whistleblower complaint said that Trump sought to use military assistance for Ukraine as leverage to push the newly elected Zelenskiy to launch an inquiry into the 2020 Democratic hopeful.

Late on Thursday, House investigators released a cache of text messages that showed top US diplomats encouraging Zelenskiy to conduct an investigation linked to Biden's family in return for granting a high-profile visit with Trump in Washington. (AP)

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