Democrats have largely held their Ukraine-based impeachment inquiry behind closed doors because their attempt at a public investigation of President Trump backfired on them, according to Sen. James Lankford, R-Okla.

Lankford told Brian Kilmeade on Fox News Radio's "The Brian Kilmeade Show" the officials Rep. Adam Schiff, D-Calif., and his committee have called to testify at the closed-door hearings are also just indirect witnesses to Trump's conduct.

"Democrats have been focused on impeachment from Day One," he said, pointing to press coverage invoking the possibility in January 2017. The senator went on to predict that the president would be impeached by the House of Representatives at some point.

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"They had 100 Democrats that were locked in -- all about Russia," he said. "They've now shifted over to Ukraine and they're all focused on that. And, they've learned their lesson: They did the Russia investigation in public so now they're doing this one in private and they're going to try a totally different tactic to try to do everything behind closed doors."

Lankford accused Democrats of only releasing information from the hearings that is intended to help their cause -- but claimed the testimony thus far is not as damning as it is portrayed.

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"The testimony that they have is pretty clear," he said. "The folks that are coming forward are their quote-unquote 'best folks' that they're going to bring forward first. All of these folks are saying, 'well, we 'perceived' the president wanted this'."

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Lankford said the only witness thus far with first-hand knowledge of Trump's intentions in regard to Ukraine was Ambassador to the European Union Gordon Sondland.

Sondland said Trump was in a foul mood when he discussed Ukraine. According to Kilmeade, the president effectively told the diplomat, "Just tell them to do what [Ukraine President Volodymyr Zelensky] ran on."

Kilmeade and Lankford agreed Zelensky's campaign was centered around quashing corruption in Kiev.

Lankford added it is not unusual for presidents to have unofficial emissaries traveling around the world apart from the State Department staff. In this case, Lankford said, former New York City Mayor Rudy Giuliani is simply one of those unofficial figures that presidents often have in their circle "behind the scenes."