Democrats in the US House of Representatives are likely to subpoena Donald Trump's former National Security Advisor John Bolton and continue their investigation into the Republican president, the head of the House Judiciary panel said on Wednesday.

House Judiciary Committee Chairman Jerrold Nadler, speaking to CNN, said "I think it’s likely, yes" that their inquiries would continue and that a subpoena for Bolton was also "likely".

After a 12-day Senate trial, senators are set to vote on convicting or acquitting the president on the two articles of impeachment passed by the House in December. As a supermajority of 67 votes is needed to remove the president from office, and with Republicans currently holding 53 seats in the 100-member chamber, the president's acquittal is considered all but certain. Trump has denied any wrongdoing.

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House investigators, who launched the impeachment inquiry on September 24, allege the president abused his power by withholding congressionally approved military aid and a White House visit from Ukraine as part of a pressure campaign seeking investigations into political rival Joe Biden and his son Hunter. They then say the president obstructed Congress by blocking requested documents and witness testimony from his administration as the House Intelligence Committee conducted their investigation.

The president has dismissed the charges as a "hoax". There is no evidence of wrongdoing by the Bidens.

The House investigators had previously sent a request for Bolton to testify during their inquiry but did not subpoena him when he failed to comply, in an attempt to avoid a lengthy legal battle that could delay the impeachment proceedings for months.

Bolton, who was forced out in September, later said he would testify if subpoenaed in the Senate trial. Experts say the statement would make it difficult for Bolton to resist a subpoena from the House.

Democrats had pushed to allow new evidence in the Senate proceedings, which could have included subpoenaing witnesses and documents, but their efforts were blocked by a 51-49 vote on Friday. Only two Republicans broke from party ranks and voted in favour of allowing new evidence.

Revelations during the trial

Reported revelations in a draft manuscript of Bolton's upcoming book, which emerged during the Senate trial, further fuelled the Democratic push for his testimony.

Bolton wrote, according to the New York Times, that Trump had instructed him in May of 2018 to call the president of Ukraine, Volodymyr Zelenskyy, to ensure that he would meet with the president's personal lawyer, Rudy Giuliani, who was leading the push for the politically motivated investigations. The directive came during a meeting in which Giuliani, White House acting Chief of Staff Mick Mulvaney, and White House lawyer Pat Cipollone were in attendance.

Bolton, according to the newspaper, had also recounted an August exchange in which Trump told him to continue to withhold the aid to Ukraine until officials in the country agreed to the desired investigations into the Bidens. The report undercut what had been Trump's legal team's main defence - that withholding the aid was unrelated to a push for the investigations.

The White House has since said Bolton's book, which he submitted for required review, contained classified information and could not be published without revisions.