House Intelligence Committee Chairman Adam Schiff said on Monday that congressional Democrats would not go to court to compel witnesses to testify in the impeachment inquiry into President Donald Trump.

Instead, Schiff said, House Democrats will use the White House's stonewalling and lack of compliance to add to potential articles of impeachment.

Charles Kupperman, the former deputy national security adviser, defied a House subpoena to testify to Congress on Monday.

Schiff's statement is a big sign that House Democrats likely have all the information they need to move forward on impeachment, and any additional testimony is icing on top.

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The chairman of the House Intelligence Committee said on Monday that congressional Democrats would not go to court to compel witnesses to testify in an ongoing impeachment inquiry against President Donald Trump.

Democratic Rep. Adam Schiff said he viewed the White House's stonewalling and refusal to allow witnesses to comply with congressional subpoenas as potential evidence of Trump's guilt. "If this witness had something to say that would be helpful to the White House, they would want him to come and testify," Schiff said. "They plainly don't."

Schiff's comments came after Charles Kupperman, the former deputy national security adviser, followed the White House's directive to ignore a subpoena to testify before lawmakers on Monday.

The California Democrat's revelation is also a big sign that Democrats believe they have enough information through other witnesses to impeach Trump over his dealings with Ukraine, particularly following damning testimony from Bill Taylor, the US's chief envoy to Ukraine, last week.

"We are not willing to let the White House engage us in a lengthy game of rope-a-dope in the courts, so we press ahead," Schiff told reporters, according to The New York Times.

Taylor, a career diplomat and war veteran, directly implicated the president in ordering the US to freeze military aid to Ukraine unless Ukrainian President Volodymyr Zelensky acceded to his demands for political investigations into his rivals. He testified that shortly after he became the US's acting ambassador to Ukraine in June, he realized there were two channels through which US policy toward Ukraine was conducted.

The first was an official one, spearheaded by Taylor. The second was an unofficial one led by Trump's personal lawyer Rudy Giuliani, Energy Secretary Rick Perry, the US ambassador to the EU Gordon Sondland, and the US's special representative to Ukraine Kurt Volker, who has since resigned.

The official channel, Taylor said, was focused on long-standing US goals vis-à-vis Ukraine, like fending off Russian aggression and combating widespread corruption within the country. But the unofficial channel, he said, zeroed in on one objective: getting Ukraine to help Trump's personal political motivations.

Taylor detailed how officials told him that Trump not only held up military aid to Ukraine but also refused to meet with Zelensky at the White House until the Ukrainian president publicly pledged to investigate Trump's political rivals and the FBI's investigation that found Russia intervened in the 2016 election to help Trump.

Recalling a September 1 conversation, Taylor said Sondland told him "everything" was dependent on Zelensky launching those investigations, including security assistance. Sondland told Taylor that Trump wanted Zelensky "in a public box" by announcing the investigations in an interview on American television, Taylor said.

Last week, The Washington Post also reported that shortly before that conversation between Taylor and Sondland, the US withdrew its recommendation to restore certain trade privileges to Ukraine. The report added fuel to the controversy because it was the first indication that Trump's pressure campaign against Ukraine may have extended to more than withholding military aid.

Taylor's testimony corroborated that given by multiple current and former government officials, including Fiona Hill, the White House's former senior director for Russian and Eurasian affairs; Marie Yovanovitch, the former US ambassador to Ukraine; and Michael McKinley, a former top State Department deputy.

This week, several additional witnesses are scheduled to testify to lawmakers and provide additional details to help Democrats flesh out the extent to which Trump's shadow foreign-policy campaign went.

The White House has tried to stem the flow of damaging revelations about how the president and his allies leveraged US policy and taxpayer dollars to force a foreign government to deliver dirt on his political rival. But their efforts have been stymied by the tide of career officials like Taylor who stepped forward to testify to lawmakers — often in defiance of the administration's orders — about what they witnessed while serving under Trump.

Trump has so far projected confidence that he would be let off the hook by the Republican-led Senate if he were impeached. But privately, the president's allies are working around the clock to win the "messaging war" over impeachment. The New York Times reported on Friday that White House insiders were now planning to create a war room and add communications aides dedicated to fighting impeachment, a sign that Trump may finally be grasping the precariousness of his position.