Democrats have angrily dismissed Republican calls for a witness swap at Donald Trump’s impeachment trial that would put Joe Biden’s son on the stand.

They rejected calls for Hunter Biden to give evidence in the US Senate trial in return for the White House agreeing to testimony from former national security adviser John Bolton.

“This isn’t like some fantasy football trade. Trials aren’t trades for witnesses,” said Congressman Adam Schiff, the lead House prosecutor.

Mr Trump is accused of abusing his presidential power to pressure the Ukrainian government to launch a probe into Mr Biden, whose son Hunter held a lucrative board position with a Ukrainian gas firm.

Campaigning in Iowa, Mr Biden, the frontrunner for the Democratic Party’s presidential nomination, insisted there would be no testimony trade-off.

“The reason I would not make the deal, the bottom line is this is a constitutional issue,” said Mr Biden.

“We’re not going to turn it into a farce or some kind of political theatre. They’re trying to do that. I want no part of that.”

Senate Minority Leader Chuck Schumer said that a trade was “off the table”.

Mr Biden has emphatically denied claims pushed by the president that he intervened to land his son the boardroom role.

Defending his son, he said: “There’s nobody that’s indicated there’s a single solitary thing he did that was inappropriate or wrong — other than the appearance. It looked bad that he was there.”

In testimony to the House of Representatives, Mr Bolton likened the political pressure allegedly put on Ukraine by Mr Trump to a “drug deal”, but he has refused to give evidence to the Senate without a subpoena.

The witness wrangling came after the House Democratic impeachment managers opened their case against Mr Trump in the Senate, with Mr Schiff accusing the president of using Ukraine in a political plot to “cheat” the results of the next election.

“If not remedied by his conviction in the Senate, and removal from office, President Trump’s abuse of his office and obstruction of Congress will permanently alter the balance of power among the branches of government,” he said in his opening remarks.

“The president has shown that he believes that he’s above the law and scornful of constraint.”

Mr Trump will become the first president to attend the annual anti-abortion “March for Life” gathering in Washington tomorrow. The rally comes weeks before the US Supreme Court hears its first major abortion case since the addition of two justices appointed by Mr Trump.