President Donald Trump is seething over an impeachment inquiry into his conduct after Democrats subpoenaed the White House about contacts with Ukraine and he signaled his administration would not cooperate.

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

"This is a fraud against the American people!" he tweeted.

The inquiry reached deeper into the White House when the House sent a letter Friday to Trump's acting chief of staff, Mick Mulvaney, informing him that the White House was being subpoenaed for documents it had refused to produce. The move capped a tumultuous week that widened the constitutional battle between the executive branch and Congress and heightened the political standoff with more witnesses, testimony and documents to come.

Trump received support from Secretary of State Mike Pompeo, who dismissed questions about Trump's attempts to push Ukraine and China to investigate Democratic rival Joe Biden as a "silly gotcha game."

"The president has every right to have these set of conversations," Pompeo told reporters while traveling in Greece. He insisted the administration's foreign efforts were reasonable, responsible and necessary to target graft, ensure aid is spent properly and protect American democracy.

"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.

It is illegal to solicit campaign help from a foreign government.

Trump, who has described his conduct as "perfect," said Friday he would formally object to Congress's impeachment inquiry, even as he acknowledged that Democrats "have the votes" to proceed. Still, he predicted such a move would hurt them politically.

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

Democrats accused Trump of speeding down "a path of defiance, obstruction and cover-up." They warned that defying the House subpoena would in itself be considered "evidence of obstruction" and a potentially impeachable offense.

Lawmakers are focused on Trump's efforts to push Ukraine investigate former Vice President Biden and his son Hunter. A whistleblower complaint said that Trump sought to use military assistance for Ukraine as leverage to push President Volodymyr Zelenskiy to launch an inquiry into 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, Reps. 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.

The White House has planned to send 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. White House press secretary Stephanie Grisham derided the subpoena as coming from a Democratic "kangaroo court."

When Pelosi announced that the House was initiating the inquiry, she didn't seek the consent of the full chamber, as was done for impeachment investigations into Presidents Richard Nixon and Bill Clinton. Pelosi insisted the House is well within its rules to conduct oversight of the executive branch under the Constitution and it is proceeding at a rapidly escalating pace.

Democrats also have sent a separate extensive request for documents to Vice President Mike Pence focused on his contacts with Ukraine. Pence spokeswoman Katie Waldman dismissed the demand, saying that "it does not appear to be a serious request" given its wide scope.

The House has also subpoenaed Pompeo, while the investigation has proceeded without their assistance. Pompeo said the State Department had responded to the subpoena, but he did not say what that response was. He had faced a Friday deadline to hand over the documents, but he suggested that he had not and would instead move to comply at his own pace.

"We'll work through that process," said Pompeo, a former congressman who was relentless in pursuing investigations into the attack on U.S. facilities in Benghazi, Libya, during the Obama administration.

Late Thursday, House investigators released a cache of text messages that showed top U.S. 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 has argued that, in his conversations, he was pushing for Ukraine to investigate corruption, not trying to undermine Biden, who could be his 2020 presidential election opponent. Trump also publicly called on China to investigate the Biden family.

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.

Associated Press writers Matthew Lee, Lisa Mascaro, Mark Sherman and Jonathan Lemire contributed to this report.