President Trump suggested his White House will cooperate with the House Intelligence Committee if it follows through with a subpoena directed at 1600 Pennsylvania Avenue, even as he attacks Democrats leading the impeachment probe and the whistleblower. Mr. Trump made the comments during a White House press conference Wednesday with Finnish President Sauli Niinistö.

"Well, I always cooperate. This is a hoax," the president said in the stately White House East Room. "This is just continuation of what's been playing out, John, for the last - since my election I would say. Probably — we'll find out soon but probably even before my election. This is a fraudulent crime on the American people but we will work together with shifty Schiff and Pelosi and all of them and we'll see what happens because we did absolutely — I had a great call with the president of Ukraine."

Three House committee chairs are readying a subpoena for the White House to demand key documents related to the July 25 phone call between Mr. Trump and the Ukrainian president. This subpoena is part of the impeachment inquiry into whether Mr. Trump jeopardized national security during the call by pressing Ukraine to investigate former Vice President Joe Biden, a political rival.

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During the press conference, the president has continued to double down on his attacks against House Intelligence Committee Chairman Adam Schiff and the whistleblower as the impeachment probe testifies. He threatened to sue a number of people, echoing threats from his personal attorney Rudy Giuliani.

In the Oval Office Wednesday ahead of the press conference, the president suggested Schiff might be guilty of treason.

"He should be forced to resign from Congress, Adam Schiff, he's a lowlife," Mr. Trump said in the Oval Office ahead of the press conference, as Niinistö looked on.

In a perplexing statement, Mr. Trump claimed Schiff couldn't even hold Secretary of State Mike Pompeo's "blank strap."

"He should resign from office in disgrace, and frankly they should look at him for treason," Mr. Trump said of Schiff.

The president has spent the last few days on Twitter attacking Schiff, as well as the whistleblower who sparked the formal impeachment inquiry. Mr. Trump told reporters Monday his administration is trying to find out about the anonymous whistleblower. He has said the person who "gave the whistleblower the information" is "almost a spy," and mused, "the spies and treason — we used to handle it a little differently than we do now."

On Wednesday, Mr. Trump said he didn't care about concerns that the whistleblower's safety could be in jeopardy. A handful of Republicans, including Iowa Senator Chuck Grassley, have emphasized the importance of protecting the whistleblower's identity and safety. Meanwhile, Mr. Trump and his allies have been working to undermine the whistleblower's credibility.

President Donald Trump speaks during a meeting with Finnish President Sauli Niinisto in the Oval Office of the White House, Wednesday, Oct. 2, 2019, in Washington. Evan Vucci / AP

House Democrats are investigating whether the president invited foreign governments to interfere in U.S. elections.

During the press conference, Finland's president urged Mr. Trump to keep the great democracy America holds dear.

"Mr. President, you have here a great democracy. Keep it going on," he said.

Asked to elaborate on that statement, Finland's president repeated what he said.