President Donald Trump on Thursday again invited foreign interference in a U.S. presidential election by publicly calling on China to investigate Democratic political rival Joe Biden, the kind of request that has already triggered an impeachment inquiry in Congress.

The Republican president told reporters he believed both China and Ukraine should look into 2020 presidential hopeful Biden and his businessman son Hunter.

“And by the way, likewise, China should start an investigation into the Bidens. Because what happened in China is just about as bad as what happened with Ukraine,” Trump said.

Questioned about whether he had asked Chinese President Xi Jinping to investigate, Trump said: “I haven’t, but it’s certainly something we can start thinking about.”

The Chinese Embassy in Washington did not immediately respond to a request for comment.

Trump’s appeal to China was particularly striking given that Washington and Beijing are locked in a bitter trade war which has damaged global economic growth. They are due to hold another round of talks in the United States next week.

The remarks stiffened the resolve of Democrats in the House of Representatives to push ahead with a probe into whether Trump should be impeached following the release last week of an anonymous whistleblower’s report, which said that in July Trump asked Ukraine to investigate the Bidens.

Hunter Biden was on the board of a Ukrainian gas company. It was not immediately clear what business Hunter Biden has conducted in China, if any.

Biden’s deputy campaign manager Kate Bedingfield said in a statement that Trump’s comments showed he was “desperately clutching for conspiracy theories that have been debunked” and “now, with his administration in free-fall, Donald Trump is flailing and melting down on national television.”

Trump froze $400 million in U.S. aid to Ukraine shortly before he asked Ukrainian President Volodymyr Zelenskiy for the favor, prompting accusations from Democrats that he had misused U.S. foreign policy and taxpayer money for personal political gain.

IMPEACHMENT PROBE

House Republican leader Kevin McCarthy wrote to Democratic House Speaker Nancy Pelosi on Thursday asking that all efforts surrounding the impeachment inquiry be suspended until “customary rules and procedures are established”.

He asked whether she planned to hold a vote of the full House authorizing the inquiry, and whether the president’s counsel would be allowed to attend all hearings and depositions. Refusing to do these things would contradict previous impeachment procedures, McCarthy said.

The impeachment probe could lead to the approval of articles of impeachment – or formal charges – against Trump in the Democratic-controlled House. A trial on whether to remove him from office would then be held in the Republican-controlled Senate. Many Republicans, however, have shown little appetite for ousting him.

The investigation has angered Trump, whose volatile presidency survived a two-year federal probe that found Russia had intervened to help him win the White House in 2016.

Trump in July 2016 publicly encouraged Russian involvement in that year’s presidential election campaign concerning cyber-espionage targeting Democratic candidate Hillary Clinton. Trump, referring to a controversy surrounding Clinton’s emails, said at that time: “Russia, if you’re listening, I hope you’re able to find the 30,000 emails that are missing.”

Nicholas Burns, a former U.S. ambassador who served under both Republican and Democratic administrations, said Trump’s comments on Thursday were a political mistake.

“In my view, these are impeachable offenses. The president has abused his power in openly inviting two governments to intervene in our election,” Burns said on CNN.

Democratic Senator Chris Van Hollen urged Republicans to turn against Trump.

“Russia. Ukraine. China. How far will Trump really go to sell out America’s sovereignty to stay in office? We already know Russia will go after our democracy again. Have my Republican colleagues really not seen enough of Trump’s shameful, illegal behavior? What will it take?”

A longtime U.S. diplomat who served as Trump’s special envoy for Ukraine spoke to congressional committee staff and members on Thursday as part of the Democratic-led impeachment probe.

Kurt Volker resigned as special representative for Ukraine negotiations on Friday, the day after the public release of the whistleblower complaint that described him as trying to “contain the damage” from efforts by Trump’s lawyer Giuliani to press Ukraine to investigate Democrats.

Wednesday was an emotional day in the impeachment inquiry when Trump railed at journalists during a news conference, resorted to the use of an expletive on Twitter and called the investigation “a hoax and a fraud” while pledging to cooperate.