President Trump held an unscheduled news conference Wednesday before leaving Davos, Switzerland, where he gave the opening speech just a day earlier at the World Economic Forum.

Mr. Trump used his remarks Wednesday to laud his administration's "new approach" to the economy, which he said had allowed the country to thrive and flourish "like never before" by slashing taxes and regulations. He also renewed his criticism of the ongoing impeachment trial against him in Washington as a "hoax," and once again defended the phone call with Ukraine's leader at the heart of that trial as "perfect."

It wasn't clear what prompted the president to call the impromptu news conference before his departure from Davos. He faced questions from reporters during his visit over the ongoing impeachment trial on Capitol Hill in Washington. Mr. Trump answered a question on it Tuesday by dismissing the proceedings as a "disgraceful" hoax.

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He came to the podium in Davos at about 6:30 a.m. (Eastern) on Wednesday, and began his news conference by reiterating the points he made in his opening speech on Tuesday, lauding America's strong economy and job creation.

On the economy and trade

"We are now by far the biggest economy in the world," Mr. Trump said, noting that there had been some predictions that China could eclipse the U.S. in that ranking. "We're an economic powerhouse like, actually, we've never been."

He said he had had important meetings with a World Trade Organization representative in Davos about the "unfair" trade relations with China and other nations. He said the WTO had been "very unfair to the United States for many years," but blamed his predecessors for "letting that happen."

Mr. Trump said his administration would be working with the head of the WTO on a "whole new structure," and he bemoaned the advantages that his administration says China, in particular, has had under WTO rules.

The president then invited WTO Director-General Roberto Azevedo to the podium, who confirmed that he would be speaking with the U.S. and other members about possible changes, which agreed were necessary to reform the organization. Mr. Trump said Azevedo and WTO representatives would be in Washington "this week or next week" to begin the discussions.

"I think we're coming into the new year with a lot of positive energy in the economy" Mr. Trump's top economic adviser Larry Kudlow said after stepping to the podium. He touted the trade deals reached with China, Japan, Mexico and Canada by the White House over the last year, saying "they are going to add a lot to growth" in the coming months.

Mr. Trump came back to the podium to say his administration would soon engage in trade talks with the European Union, which he called a tougher negotiating partner than China. He repeated his assertions from Tuesday that companies from Asia and Europe had said they were planning significant new investments in the U.S.

On the impeachment trial

Asked again about the impeachment trial, Mr. Trump said it was nothing more than "a hoax, a con-job by (Representative Adam) Schiff" and "the greatest witch hunt." Schiff, the chairman of the House Intelligence Committee, is one of the seven Democrats serving as impeachment managers.

"I'm gonna head back and I'll be watching it, but it will really be up to the Senate," he said when asked by CBS News' Paula Reid if he wants witnesses to testify during the trial.

Later in the news conference Mr. Trump returned to the topic, once again defending his "perfect" July phone call with Ukrainian President Volodymyr Zelensky — the call at the heart of allegations of abuse of power against Mr. Trump, who pressured Ukraine to investigate his political rival Joe Biden.

"The President of Ukraine said it was perfect. The Foreign Minster of Ukraine said it was perfect," Mr. Trump insisted, reiterating his denial of claims that he held up congressionally-approved U.S. financial aid to the country in an effort to have Ukrainian leaders announce a probe into Biden's financial dealings in the country.

He said he would "love" to have U.S. Secretary of State Mike Pompeo, acting White House chief of staff Mick Mulvaney, and Energy Secretary Rick Perry testify before Congress, as Democrats have demanded, but said it was not possible due to "U.S. national security."

Mr. Trump told reporters said he would also rather have former national security adviser John Bolton appear before lawmakers, but conceded Bolton's testimony could hinder his diplomatic efforts. Bolton said earlier this month he would testify in the Senate trial if subpoenaed.

"He knows some of my thoughts, he knows what I think about leaders. What happens if he reveals what I think about a certain leader and it's not very positive, and I have to deal on behalf of country?" Mr. Trump said. "It's going to be very hard. It's going to make the job very hard."

The president said he would prefer to "go the long way" with the Senate's trial, a statement that puts him at odds with his own legal team. Mr. Trump's attorneys urged the Senate in a legal brief earlier this week to "speedily reject" the articles of impeachment and acquit Mr. Trump.

"It's horrible for our country. Our country has to get back to business," Mr. Trump said of the impeachment.

He also suggested that the U.S. received permission from Ukraine's government to publish the White House's partial transcript of the July phone call, something Ukrainian officials have denied. Ukrainian officials have told CBS News that White House officials requested permission to publish some transcription of the call, but that it was never given in the few hours they had to reply before the partial transcript was released.

Mr. Trump called the Democrats leading the impeachment trial "major sleazebags."

On Greta and the environment

On Tuesday Mr. Trump took aim at critics of his environmental policies — without specifically naming teen activist Greta Thunberg — calling for "the prophets of doom" and their warnings of an "apocalypse" to be rejected in favor of optimism.

A reporter asked Mr. Trump Wednesday about his veiled criticism of Thunberg, and the broader criticism of his administration's easing of environmental standards. The president said "some people" were demanding changes that were "unrealistic," before lauding the cleanliness of U.S. air and water.

"We are doing better right now in terms of cleanliness, in terms of numbers" Mr. Trump said, suggesting the teenage activist focus her ire instead on "other countries," which he accused of sending waves of trash crashing onto U.S. Pacific shores.