The demand for documents capped a tumultuous week that widened the constitutional battle between the executive branch and Congress and sharpened the political standoff with more witnesses, testimony and documents to come.
Trump said he would formally object to Congress about the House impeachment inquiry, even as he acknowledged that Democrats "have the votes" to proceed. They'll be sorry in the end, he predicted.
"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" and a potentially an impeachable offense.
Lawmakers have made Trump's request last summer that Ukraine investigate former Vice President Joe Biden the centerpiece of the probe. A whistleblower complaint said that Trump sought to use military assistance for Ukraine as leverage to push President Volodymyr Zelenskiy 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, 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.
Fighting the inquiry, the White House was expected to send a letter to House Speaker Nancy Pelosi arguing that Congress could not mount its 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."
But Pelosi insisted the House is well within its rules to conduct oversight of the executive branch under the U.S. 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.
When Pelosi recently 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 former Presidents Richard Nixon and Bill Clinton. But it is proceeding at a rapidly escalating pace.
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 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.
Pelosi has sought to avoid a vote on the impeachment probe for the same reason she resisted, for months, liberal calls to try to remove the president: It would force moderate House Democrats to make a politically risky vote.
The White House, meanwhile, is trying to force the question on Democrats, as it seeks to raise the political cost for their impeachment investigation and to animate the president's supporters ahead of the 2020 election.
Two days after telling reporters, "Well, I always cooperate," Trump struck a different note on cooperating with the House probe. "I don't know," he said. "That's up to the lawyers."
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.
Rudy Giuliani, the president's personal attorney, dismissed the entire premise of the impeachment inquiry, which is centered on Trump asking Ukraine to investigate Biden.
"The president was not tasking Ukraine to investigate a political opponent," Giuliani told The Associated Press on Thursday. "He wanted an investigation into a seriously conflicted former vice president of the United States who damaged the reputation of the United States in Ukraine."
Democrats have sought to use their declared impeachment investigation to bolster their case to access all sorts of documents from the administration, most recently secret grand jury information that underpinned special counsel Robert Mueller's report on Russian interference in the 2016 election.
Where courts have generally required congressional oversight requests to demonstrate a legitimate legislative purpose, impeachment requests could be wide-ranging.
Democrats have already won some early court battles. It is unclear if Democrats would wade into a lengthy legal fight with the administration over documents and testimony — or if they would just move straight to considering votes on articles of impeachment.