A new witness in the impeachment investigation of President Donald Trump rocked the White House Tuesday with testimony that he personally witnessed officials pressuring Ukraine to help Trump politically.

National Security Council Ukraine expert Lieutenant Colonel Alexander Vindman was to tell the House inquiry that he twice reported concerns about improper White House efforts to get Kiev to open investigations designed to help Trump politically.

“How many more Never Trumpers will be allowed to testify about a perfectly appropriate phone call,” he asked. “Supposedly, according to the Corrupt Media, the Ukraine call ‘concerned’ today’s Never Trumper witness. Was he on the same call that I was? Can’t be possible!”

Republicans, mobilised to undercut Vindman’s credibility, questioning his loyalty by noting he immigrated to the United States from the Soviet Union at the age of three, and suggesting he is part of an effort by the US national security bureaucracy to undermine Trump.

In explosive prepared testimony, Vindman said he personally listened to Trump pressure Ukraine President Volodymyr Zelensky in a July 25 phone call.

His testimony, released late Monday, offers some of the strongest evidence yet for accusations that Trump abused his presidential powers and broke election laws to gain Kiev’s support for his re-election effort next year.

“Donald Trump is innocent. The deep state is guilty,” said Republican lawmaker Matt Gaetz, one of the president’s most strident defenders in Congress.

Republicans have argued that nine previous witnesses only knew of the call’s contents second-hand, although a heavily edited official “transcript” of the call appears to match testimony that Trump pressured Zelensky for political reasons.

Vindman says in the prepared testimony that a senior US diplomat close to Trump, ambassador to the European Union Gordon Sondland, was the first person he witnessed pressing Ukraine for the investigations, in a July 10 meeting with Ukraine national security official Oleksandr Danylyuk.