Complex sentence: Lichtman said Comey's firing gave fodder to the debate that Trump had engaged in obstruction of justice, and he emphasized at that point in time he was not calling for impeachment itself, rather an impeachment investigation.
Split into simpler sentences: Lichtman said Comey's firing gave fodder to the debate that Trump had engaged in obstruction of justice. He emphasized he was not calling for impeachment itself, rather an impeachment investigation.