President Trump on Thursday publicly called on China to investigate a political rival, former Vice- President Joseph R. Biden Jr, in an extraordinary presidential request to a foreign country for help that could benefit him in the 2020 election.

“China should start an investigation into the Bidens,” Trump said on Thursday as he left the White House to travel to Florida where he was expected to announce an executive order on Medicare.

The call for China to investigate Biden and his son Hunter’s dealings there came as the first witness appeared on Capitol Hill to be interviewed by House investigators as part of an impeachment inquiry into the President’s request in a phone call for help from President Volodymyr Zelensky of Ukraine.

Trump has defended his conversation with Zelensky as “perfect” even after a reconstructed transcript of the call was released that showed him seeking help from Ukraine in investigating the Bidens. And he doubled down on his request on Thursday.

“I would say that President Zelensky, if it were me, I would recommend that they start an investigation into the Bidens,” Trump said. “Because nobody has any doubt that they weren’t crooked.”

These requests, which critics argue are an abuse of presidential power, echo comments Trump made as a presidential candidate in 2016 for Russia to release missing emails of his political opponent, Hillary Clinton. Trump made the comments about China ahead of the latest round of trade talks.

“We’re going to have a meeting with them, we’ll see,” Trump said of the talks. “I have a lot of options on China. But if they don’t do what we want, we have tremendous power.”

In calling for China to investigate the Bidens, Trump referred to a business deal Hunter Biden was in that involved a fund drawing from investment from the Chinese government-owned Bank of China.

The fund was announced in late 2013 — days after Hunter Biden and one of his daughters flew to China from Japan aboard Air Force Two with the vice-president, who was in the midst of a diplomatic mission to calm rising tensions in the region, warning Chinese leaders not to use fighter jets to enforce an air defence zone created by Beijing over contested waters.

The conservative author Peter Schweizer claimed that Hunter Biden used the trip to secure a deal with the Bank of China. That allegation has been echoed by Trump’s allies, and by the President himself on Thursday.

But a lawyer for Hunter Biden has said that he did not conduct any business related to the China investment fund on that trip, and that he was never an equity owner in the fund while his father was vice president.