House Republicans plan to sharpen their focus on allegations of Ukraine meddling in the 2016 presidential election during the open phase of impeachment inquiry hearings, voicing frustration after top diplomats said Wednesday they had no knowledge of the issue.

A senior Republican official told Fox News on Thursday that the issue of Ukrainian election meddling would be a “theme” of questions asked by GOP members on the House Intelligence Committee moving forward.

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During the first public hearing on Wednesday, Republican members ventured into that territory when they asked State Department official George Kent and acting ambassador to Ukraine Bill Taylor questions about Alexandra Chalupa—a former Democratic National Committee consultant who allegedly had meetings during the 2016 campaign with officials at the Ukrainian Embassy in Washington to discuss incriminating information about Trump campaign figures.

House Intelligence Committee Ranking Member Devin Nunes, R-Calif., asked Taylor a series of questions related to Ukraine and Chalupa, reminding the diplomat that in his closed-door deposition, he testified he was unfamiliar with Chalupa’s actions at the Ukrainian embassy.

“It is correct that I had not known about this before,” Taylor said. He also confirmed that he was disappointed by the allegations when they were presented to him at the deposition.

Kent, meanwhile, testified that he saw no factual basis to support allegations of Ukraine interference in the 2016 election.

“Democrats have all these people and officials they are parading as witnesses, who are so concerned about Trump and his people making inquiries about this, but they all say they had no idea of these reports, and deny any type of Ukrainian election meddling,” a GOP source said. “We will continue to press this thing forward.”

Fox News, over the weekend, first reported on the Republicans’ proposed witness list for the upcoming public impeachment hearings, which included Chalupa.

“Given President Trump’s documented belief that the Ukrainian government meddled in the 2016 election to oppose his candidacy, which forms the basis for a reasonable desire for Ukraine to investigate the circumstances surrounding the election and any potential Ukrainian involvement, Ms. Chalupa is a prime fact witness who can assist Congress and the American public in better understanding the facts and circumstances surrounding Ukrainian involvement in the 2016 election,” Nunes wrote over the weekend when proposing Chalupa as a witness.

Chalupa was first brought into the conversation in January 2017, after Politico published a report exposing her as a DNC operative, who once worked in the Clinton White House and during the 2016 campaign met with officials in the Ukrainian Embassy in an effort to expose ties between former Trump campaign chairman Paul Manafort and Russia.

In 2017, then-Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, penned a letter to former Deputy Attorney General Rod Rosenstein, citing that same Politico report and questioning Chalupa’s alleged actions, which he said seemed to show that she “was simultaneously working on behalf of a foreign government, Ukraine, and on behalf of the DNC and Clinton campaign, in an effort to influence not only the U.S. voting population but U.S. government officials.”

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Grassley also questioned why Chalupa was not forced to register as a foreign agent under the Foreign Agents Registration Act (FARA)--a move her attorney, at the time, told Fox News was "inapplicable" to her actions.

Chalupa's attorney Conrad Nowak, in 2017, told Fox News that “Chalupa was nothing more than an individual involved in ethnic relations, not unlike countless other ethnic and heritage communities throughout the United States.” Nowak also said, at the time, that "if there was anything to this Ukraine red herring, we would’ve heard about it a long time ago.”

At the time, the DNC told Fox News that the Ukraine narrative was simply an effort to "distract" from the Russia probe looming over the Trump White House.

But Republicans are homing in on Chalupa, referring back to the Politico report and past GOP efforts to investigate allegations.

“The Democrats will have a hard time continuing to dismiss the Chalupa affair as a conspiracy theory, seeing as Chalupa admitted to it and Ukrainian officials confirm it,” a senior GOP source involved in the impeachment hearings told Fox News. “It’s a problem for them because it shows Ukrainians were cooperating with the Democrats in election meddling, and therefore Trump was justified in making inquiries about it.”

But Democrats are pushing back at Republicans’ strategy and their claims.

“First, there’s not a shred of evidence of Ukrainian election meddling,” Rep. Eric Swalwell, D-Calif., a member of the House Intelligence Committee, told Fox News. “Second, even if Donald Trump irrationally believed that, Tom Bossert, his former senior homeland security official, has said he told Trump that this belief was nonsense.”

It is highly unlikely Intelligence Committee Chairman Adam Schiff, D-Calif., and Democrats on the panel will approve Republicans’ request to have Chalupa appear as a witness. Schiff and Democrats, as per a newly passed resolution governing the impeachment inquiry, have the final stamp of approval as to who can testify as part of the formal inquiry.

At the center of the impeachment inquiry, which began in September, is President Trump’s July 25 phone call with Ukrainian President Volodymyr Zelenksy. That call prompted a whistleblower complaint to the intelligence community inspector general, and in turn, the impeachment inquiry in the House.

On the call, Trump pressed Zelensky to open an investigation into Ukrainian election meddling in the 2016 presidential race (related to an allegation separate from the Chalupa meeting) and into former Vice President Joe Biden and his son Hunter Biden’s business dealings in Ukraine.

The president’s request came after millions in U.S. military aid to Ukraine had been frozen, which Democrats and witnesses have cited as a quid pro quo arrangement.

Zelensky, though, has said he felt no pressure during the call. The White House has maintained no wrongdoing, and the president has repeatedly said the call was “perfect,” arguing that it contained “no quid pro quo.” The plot thickened when Taylor testified Wednesday that a staffer overheard a phone call where Trump discussed such "investigations," the day after his controversial phone call.

The Associated Press reported Thursday that a second U.S. Embassy staffer in Ukraine also overheard the call.

A Republican source told Fox News that GOP members of the Intelligence Committee will likely question subsequent witnesses on the Ukraine meddling topic. Up next, on Friday, is former U.S. Ambassador to Ukraine Marie Yovanovich.