Rudy Giuliani met with disgraced Ukrainian prosecutor Yuri Lutsenko in January 2018. And during the meeting, Giuliani called President Trump to patch him in.

This is a third-hand story given by the former United States Ambassador to Ukraine Marie Yovanovitch. She did not claim to have seen this happen, only that one of her aides had been told this by a Ukrainian deputy prosecutor named Yevhen Yenin.

It's a third-hand story, so take it for what it is. But if it is true, it could prove decisive for Trump's political fate.

To date, Trump has relied on the transcript of the call between himself and Ukrainian President Volodymyr Zelensky from July 25 of this year as evidence that his request that Ukraine investigate the Bidens was just a part of his greater concern about corruption. Calls between Trump and Lutsenko, likely without the White House's knowledge and per Giuliani's arrangement, could either prove or disprove that defense.

As every reporter who's ever been butt-dialed by the president's personal attorney knows, Giuliani plays fast and loose with telecommunications security. If he looped Trump into a call without the White House's knowledge, there's little question that the Russians would have had the ability to intercept the call. If such a call took place, it's impossible to believe that anyone in the West Wing could have known about it without setting off a fire alarm.

So, if such a call actually occurred, its contents could prove that Trump was actually simply obsessed with some CrowdStrike conspiracy theory or the Bidens as they pertain to Ukrainian corruption, rather then hellbent on using foreign governments to investigate his domestic political foes.

Congress has called upon dozens of witnesses as part of their impeachment proceeding, but they must call two key witnesses to get to the bottom of this story.

First, and thanks to Yovanovitch's testimony, we know they must call Joseph Pennington to testify. All rumors about the Trump call came from Ukrainians, and seeing as Congress has limited fact-finding power over Ukraine, it needs to glean as much from Ukraine's perspective as possible. Thus, and as I have been urging Congress for weeks now, Congress must call upon Amos Hochstein, the only American citizen who seems to have advised Zelensky specifically about how to avoid Giuliani's electoral machinations. Remember, Energy Secretary Rick Perry seemed keen on coercing Ukraine into booting Hochstein from the anti-corruption advisory board of national gas firm Naftogaz. He has offered little objective reason as to why. If anyone heard of Ukrainians relaying rumors of a Trump-Lutsenko call, Hochstein seems like a good place to start.

There's a wild conspiracy theory to investigate, but it's not CrowdStrike or Burisma or the DNC. It's the rumor casually dropped into the world by Yovanovitch.