WASHINGTON (Reuters) - U.S. congressional Democrats, faced with stepped-up efforts by President Donald Trump’s Republican allies to disrupt their impeachment inquiry, are now considering calling only career government employees to testify at public hearings expected to start next month, according to congressional sources.

Democrats on the three House of Representatives committees handling the initial inquiries plan to conduct at least two more weeks of closed-door depositions with witnesses knowledgeable about dealings that Trump, his personal lawyer Rudy Giuliani and other officials have had with Ukrainian authorities including President Volodymyr Zelenskiy.

Top Trump advisers such as Giuliani, Secretary of State Mike Pompeo, acting White House Chief of Staff Mick Mulvaney and Energy Secretary Rick Perry have defied House subpoenas for documents.

But other witnesses have appeared for committee depositions despite instructions from the Trump administration to boycott such proceedings, and in some cases have provided what Democrats describe as damning evidence about Trump’s efforts to pressure Ukraine to serve his own political interests.

The Democratic-led inquiry is focused on Trump’s request in a July 25 telephone call to Zelenskiy to investigate a domestic political rival, Joe Biden, the former vice president who is a leading contender for the 2020 Democratic presidential nomination. U.S. law prohibits candidates from accepting foreign help in an election.

Televised proceedings could help Democrats stoke public support for a House vote to impeach Trump later this year, while increasing pressure on Republican lawmakers to break with Trump and support an impeachment resolution.

But after a group of Republican lawmakers threw the inquiry into chaos this week by barging into a high-security hearing room and forcing a delay in testimony from a Pentagon official, Democrats have grown more concerned about the potential for disruptions, said the sources, who were not authorized to speak on the record.

Republicans have denounced the inquiry as illegitimate and called on Democrats to hold a full House vote authorizing the investigation and to give Trump and Republican lawmakers the ability to cross-examine witnesses and subpoena their own.

EARLY STAGE

While the planning of public proceedings remains at an early stage, the sources said Democrats are inclined to avoid open testimony by Giuliani, Mulvaney, Pompeo and others.

Trump’s former national security adviser John Bolton could prove to be the exception, given closed-door testimony that he was alarmed by efforts to pressure Ukraine. But sources said it is not clear whether Bolton would be willing to speak publicly about what occurred.

Democrats are instead considering a limited number of public hearings that would feature testimony only by nonpolitical career diplomats such as acting U.S. Ambassador to Ukraine William Taylor and former U.S. Ambassador to Ukraine Marie Yovanovitch, according to the sources.

In closed-door testimony on Tuesday, Taylor told the three investigating House committees that Trump made the release of $391 million in security assistance to Ukraine contingent on Kiev publicly declaring that it would carry out investigations into Biden and a debunked conspiracy theory about the 2016 U.S. election.

Yovanovitch testified earlier that Trump ousted her from her post in Ukraine based on “unfounded and false claims” after she came under attack from Giuliani.