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(CNN) —  

Several Republican lawmakers were more “shaken” than they publicly let on by the testimony of a US official in Kiev who overheard a phone conversation between the US ambassador to the European Union and President Donald Trump, a top GOP congressional source told CNN.

David Holmes – who will testify publicly as part of the House impeachment inquiry – said in closed-door testimony last week that US Ambassador to the EU Gordon Sondland had told Trump that Ukrainian President Volodymyr Zelensky “loves your ass” and that Ukraine was going to move forward with the investigation Trump had asked Zelensky for a day earlier. Holmes said he was able to overhear the conversation due to the volume of Trump’s voice while he sat with Sondland at a restaurant in Kiev.

According to the GOP congressional source, that testimony led several GOP lawmakers to express frustration that Sondland would place a call to the President in a public restaurant, and are concerned that Holmes’ testimony was the most convincing argument for Trump’s direct involvement in the campaign to pressure Ukraine.

The House Intelligence Committee will have Holmes, the counselor for political affairs at the US Embassy in Ukraine, testifying alongside former White House official Fiona Hill on Thursday, according to the Democratic aide. The addition of Holmes means nine individuals will testify publicly as witnesses in the House impeachment probe this week.

Holmes’ testimony is making some GOP members worry about how far Sondland will go in his public testimony on Wednesday and two senior Republican sources say that some House Republicans are worried about how Sondland will handle himself at the hearing.

The sources point out that Sondland is not an accomplished diplomat and one source adds he believes Sondland was unprepared and ill fitted for the job as US ambassador to the European Union. According to multiple State Department and former State Department officials, he was not well regarded by the US diplomatic community.

House Republicans are also increasingly worried about the political fallout from the hearings overall and the impact of multiple witnesses who are career professionals.

They are especially concerned about the reaction from independent voters and suburban women voters who are watching Trump attack witnesses both on Twitter and on television.

Holmes’ statement confirmed the testimony from Bill Taylor, the top US diplomat in Ukraine, who revealed the July 26 phone call with lawmakers at his own public testimony last Wednesday, saying Holmes had informed Taylor about it after Taylor first appeared on Capitol Hill behind closed doors last month. Holmes’ testimony places Trump closer to the push for Ukraine to open an investigation into his political rival and raises additional questions about the testimony of Sondland, who is scheduled to testify publicly Wednesday.

Holmes, who had been subpoenaed to appear last Friday, explained that Sondland had placed the call to Trump, and he could hear Trump because the call was so loud on the terrace of a restaurant in Kiev where they dined with two others.

CNN’s Manu Raju and Jeremy Herb contributed to this report.