The top Ukraine expert on the national security council planned to tell Congress on Tuesday that a July phone call between Donald Trump and the Ukrainian president, Volodymyr Zelenskiy, caused him to raise an internal alarm about a suspected subversion of US foreign policy – by the White House.

The anticipated testimony before congressional investigators by Alexander Vindman, an army lieutenant colonel who was decorated with a Purple Heart after being wounded by a roadside bomb in Iraq, could weigh heavily in the impeachment inquiry against Trump.

Vindman’s planned opening testimony, describing what he felt was a shocking swerve of US policy toward Ukraine, was published late Monday by CNN and excerpted by the New York Times.

The excerpts indicate that Vindman, who planned to testify in defiance of a White House gag order, was prepared to corroborate previous, damning testimony that the Trump administration mounted a campaign to pressure Ukraine into announcing an investigation of Joe Biden, in exchange for military aid and a possible White House visit.

“I did not think it was proper to demand that a foreign government investigate a US citizen, and I was worried about the implications for the US government’s support of Ukraine,” said Vindman’s statement as quoted in the Times. “I realized that if Ukraine pursued an investigation into the Bidens and Burisma it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained.”

Burisma is a gas company on whose board Hunter Biden, the former vice-president’s son, served for five years.

The White House has denied any such effort to establish a quid pro quo with Ukraine, but against mounting evidence to the contrary Trump’s defenders have argued that the president’s conduct does not rise to the level of impeachment.

Vindman, a Ukrainian-American immigrant and the current European affairs director at the national security council, planned to indicate in his testimony that he raised an internal alarm about the shadow US policy toward Ukraine. His opening statement suggested that he might have been one of the White House sources for a whistleblower report that kicked off the impeachment inquiry.

“This would all undermine US national security,” Vindman planned to say of Trumps’s 25 July call with Zelenskiy.

The Times report quoted Vindman’s statement further: “I am a patriot, and it is my sacred duty and honor to advance and defend our country irrespective of party or politics.

“I did convey certain concerns internally to national security officials in accordance with my decades of experience and training, sense of duty, and obligation to operate within the chain of command.”

In keeping with the previous testimony of current and former officials, including the diplomat William Taylor and the Russia expert Fiona Hill, Vindman planned to testify that he was concerned about the role the president’s personal lawyer, Rudy Giuliani, who was also on the payroll of opaque interests in Ukraine, was playing in crafting US policy towards Ukraine.

Vindman also planned to contradict the embattled US ambassador to the European Union, Gordon Sondland, who returned to Capitol Hill on Monday with his lawyers to “review” his testimony of two weeks prior.

In his testimony before the impeachment investigators, Sondland, who is a hotelier and Trump major-donor, but not a diplomat, reportedly downplayed his role in making Ukraine understand what Trump needed in exchange for military aid, which had already been appropriated by Congress.

Vindman was shocked at the linkage between aid and political favors, he planned to testify, and he confronted Sondland the day Sondland spoke in a White House meeting about “Ukraine delivering specific investigations in order to secure the meeting with the president”.