WASHINGTON (Reuters) - A former Republican congressman has become a leading lobbyist and helped sack US Ambassador to Ukraine Mary Jovanovich, a US diplomat said in testimony to the removal of US President Donald Trump in the Ukraine scandal.

Catherine Croft said that while in the National Security Council for a year starting in July 2017, she had received several calls from Robert Livingstone (a former congressman) who told her that Ambassador Jovanovic should be said, which Trump also did under pressure from his lawyer. Rudy Giuliani.

The testimony revealed a previously unknown role for Livingstone in the Ukraine case. Livingstone had previously played a leading role in a Republican-led campaign to oust Democratic President Bill Clinton in the 1990s.

Catherine Croft and Christopher Anderson, the State Department's Ukraine specialist, are the latest among current and former officials to testify as part of a Democrat-led inquiry to hold Trump accountable in the House of Representatives.

In his opening speech, seen by Reuters, Anderson also referred to Giuliani's work. He said he and his colleagues watched a tweet by Giuliani before the inauguration of President Vladimir Zelinsky, claiming that "Ukraine's next leader is surrounded by enemies of President Trump." Anderson said he had sought to counter the "negative narratives" sparked by the dismissal of Jovanovic and Giuliani's comments.

Catherine Croft is among current and former officials who testified in the investigation to isolate Trump (French)

US parliamentary committees investigating the dismissal of Trump also summoned former national security adviser John Bolton to testify about a possible abuse of power by the White House master, US media reported Wednesday.

Bolton, who was dismissed in September, was invited to testify in a closed-door hearing on November 7, the Washington Post reported, as the White House sought to limit the cooperation of current and former officials with the investigation.

Bolton is reported to have previously warned of secret efforts by the US administration, particularly Giuliani, to press Ukraine to open an investigation against Democrats, including Biden.

The investigation into Trump's accountability is centered on a telephone call on July 25 in which Trump asked Ukrainian President Zelinsky to investigate his political rival, former vice president Joseph Biden, and his son Hunter, who served as director of Ukraine's Purisma Energy.Trump denies any wrongdoing.

Trump made the request to Zelinsky after blocking $ 391 million in security aid to Ukraine after Congress approved it to help Ukraine fight Russian-backed separatists in eastern Ukraine. Zelinsky agreed to Trump's request, and aid was provided later.