On Sunday, Giuliani explained he's considered a "witness" due to the speech he personally gave leading up to the Capitol siege.
"Because I gave an earlier speech [at the January 6 Trump rally], I am a witness and therefore unable to participate in court or Senate chamber," Giuliani said to ABC News' chief White House correspondent Jonathan Karl.
Leading up to the illegal rampage at the United States Capitol building, Giuliani was part of the several speakers who urged the crowd to take matters into their own hands to "stop the steal." In fact, he insisted that the crowd of pro-Trump supporters spark a "trial by combat" to address the nonexistent voter fraud.
These actions make Giuliani's liberal use of "witness" the wrong term to label himself. In reality, he's more like Trump's codefendant and could possibly face legal repercussions for his actions.
Also because of Giuliani's connection to the riots, it's likely his participation in the impeachment trials would solidify a guilty verdict.