(CNN) The top US diplomat in Ukraine, Bill Taylor, will "fulfill his duty" and return to Capitol Hill to testify publicly if asked, according to a source familiar with his thinking, making him a potential key early witness for Democrats as they shift their impeachment inquiry into a public phase.

There has not been a request yet made for Taylor to testify publicly, according to the source, and Taylor is now focused on his duties in Ukraine.

Taylor's lawyers said Wednesday night that no decision had been made.

"Ambassador Taylor has not been asked to appear and therefore has not made any decision about future testimony. He continues to perform his duties as a government official," his lawyers told CNN.

But Democrats privately say that Taylor would be a logical choice for one of the first witnesses when the House begins public impeachment hearings. He meticulously documented how he believed the White House had linked Ukraine's announcing an investigation that could help the President politically with unfreezing of US security aide and a one-on-one meeting between President Donald Trump and Ukrainian President Volodymyr Zelensky.

The House on Thursday will vote on a resolution that will formalize the rules for the impeachment inquiry and set the stage for public hearings in the House Intelligence Committee. House Intelligence Chairman Adam Schiff, a California Democrat, has declined to comment on which witnesses he will bring in or when the hearings will start, but lawmakers and aides say that a timetable where hearings begin next month before Thanksgiving is feasible, though not finalized.

"It doesn't get much more 'Top Gun' than that"

Democrats believe that Taylor, a career diplomat who has worked for administrations of both parties, will be an unassailable witness with detailed notes and an impeccable memory. They believe he's someone who will tell a story to the public authoritatively about why efforts to provide aid to Ukraine and bolster relations with that key ally were delayed amid the push by Trump to investigate the Bidens and the 2016 elections. His 15-page opening statement delivered some of the most dramatic and damning testimony that Democrats have heard thus far.

"He's rock solid, detailed notetaker and unimpeachable," said Rep. Jackie Speier, a Democratic member of the House Intelligence Committee. "Fifty years given to his country -- it doesn't get much more 'Top Gun' than that."

But the Republican strategy for the open hearings also is coming into focus. They plan to zero in on the lack of direct contact with Trump -- and contend that the diplomats' concerns were merely stating their own personal opinions. It's a key argument they'll make with Taylor, who told lawmakers he never met with Trump one-on-one, according to a source familiar with his testimony.

"A lot of that information is based off of second-, third- and even fourth-hand information," said New York Rep. Lee Zeldin, who has been a Republican spokesman following many of the depositions. Zeldin said Taylor's remarks contained "only two references in his opening statement to the Bidens," and that the reference to investigating the Bidens came to Taylor from his conversation with National Security Council staffer Tim Morrison who was told by US Ambassador to the European Union Gordon Sondland.

Former Ambassador William Taylor leaves a closed door meeting after testifying as part of the House impeachment inquiry on Capitol Hill on October 22.

Taylor, however, also testified that Sondland told him directly that "everything" was conditioned on a public announcement, and that Trump had said he wanted Zelensky "in a public box" on the investigation.

Republicans say they're ready for public hearings.

"I'm not worried about anybody in a public setting," said Pennsylvania Rep. Scott Perry, a GOP member of the House Foreign Affairs Committee who has participated in the closed depositions. "If you think the President has done something wrong, you're going to see that in the witness testimony. If you think the President hasn't done anything wrong, you're going to see that in the witness testimony."

Perry added: "I'm not worried about any of them testifying because the facts are the facts. Most of this is opinion. These people have diverging opinions -- that's their right to have it. But quite honestly, the President is within his rights to do any of these things."

Taylor is one of several witnesses whom Democrats may ask to return as they move from gathering information behind closed doors in their investigation to presenting that information to the public to make the case for impeaching a President for just the third time in US history. They're also interested in other witnesses who can corroborate the account that Trump pushed for Ukraine to open an investigation while holding up security aid — as well as to press Sondland about discrepancies they see in his account from his conversations with Taylor and White House meetings on Ukraine.

Sondland's lawyer did not reply to a request for comment when asked if he would be willing to return to testify publicly. Schiff's office declined to comment on Taylor's potential testimony.

"I don't speak Latin"

Taylor's opening statement provided a preview of what kind of witness he would be for Democrats publicly. He was unequivocal that he believed US aid to Ukraine was being held up as Trump and his personal attorney Rudy Giuliani pushed for an investigation — and he took detailed notes.

"According to Mr. Morrison, President Trump told Ambassador Sondland that he was not asking for a 'quid pro quo.' But President Trump did insist that President Zelensky go to a microphone and say he is opening investigations of Biden and 2016 election interference, and that President Zelensky should want to do this himself," Taylor testified.

While Democrats seized on Taylor's comments to say he'd laid out a quid pro quo, Taylor didn't want to say so explicitly himself during his closed-door testimony, according to multiple sources familiar with this testimony.

Taylor is seen here outside Capitol Hill on October 22.

In a previously unreported exchange, Schiff asked Taylor if the deal was essentially that Zelensky needed to announce investigations and then he would get the White House visit and security assistance, and Taylor said yes, according to the sources.

Schiff then asked, "Isn't that the very definition of a quid pro quo?"

"I don't speak Latin," Taylor responded, according to the sources. He said that was a legal conclusion — he was there to provide the facts — and he would let the lawmakers draw those legal conclusions.

Sources told CNN that when Republicans questioned Taylor, one of the things they focused on was his relationship with Morrison, who is scheduled to testify behind closed doors Thursday. While Republicans initially tried to cast them as friends, Taylor said he had never met Morrison before they both ended up in these roles, according to one source. They also tried to get at the idea that it would be improper for Morrison to convey to Taylor what Sondland said about his conversation with Trump, but Taylor explained that was Morrison's job.

Taylor did not record any of his phone calls, one source said, but he's been a note-taker his whole life and had extensive notes of his conversation with Sondland, Morrison and Deputy Assistant Secretary of State for European and Eurasian Affairs George Kent . The items that he quoted in his opening statement were quotes from his notes or from WhatsApp messages, the source said.

More witnesses likely to return

While Schiff and other committee leaders have been tight-lipped on which witnesses could make public appearance, Democrats say there will be several weeks of public hearings — and likely return witnesses.

Those hearings could begin next month, as the House Intelligence, Oversight and Foreign Affairs committees are working to complete all of their closed-door depositions — they have six scheduled for this week and several more on tap for next week. That schedule could still slip, however, lawmakers warn, as many of the depositions have yielded additional potential witnesses.

Several Democrats have said they'd been interested in hearing publicly from former US Ambassador to Ukraine Marie Yovanovitch , who was removed earlier this year after Giuliani pushed for her ouster. They're also interested in Fiona Hill , the White House's former top Russia adviser, who reported her concerns about Giuliani's Ukraine push and testified that former national security adviser John Bolton compared the effort to a "drug deal."

Lt. Col. Alexander Vindman , the National Security Council Ukraine expert who testified Tuesday that he reported his concerns about the July 25 phone call, would give Democrats a firsthand witness who was on the call between Trump and Zelensky.

Multiple sources said that Vindman could be a good witness, but he also appeared nervous when he appeared behind closed doors Tuesday. The sources said Vindman seemed a bit unprepared for the moment at first, but then he got into a groove with his responses during the question-and-answer portion of the deposition.

Morrison, who is scheduled to testify behind closed doors Thursday, could also be a key witness, as he's cited in Taylor's opening statement more than a dozen times.

And Democrats have also said they want to hear again from Sondland, who had told Taylor there was "no quid quo pro" in a text message after speaking to Trump. His testimony appeared to contradict other witnesses, particularly surrounding a July 10 meeting on Ukraine, and Democrats want him to return to clarify it.

Schiff would not say Tuesday whether he would try to bring Sondland back.

CLARIFICATION AND UPDATE: This story and its headline have been updated to reflect that Taylor will "fulfill his duty" to testify only if asked, and to add a statement from Taylor's lawyers.