Video: Trump Impeachment Trial Day 6

Senators are facing increasing pressure to call John Bolton to testify at President Donald Trump's impeachment trial.

However, Trump's lawyers on Monday brushed past the extraordinary allegations in the draft of a new book by his former national security adviser. Instead they focused on allegations of corruption in Ukraine and legal arguments for acquittal.

The draft of Bolton's book says Trump told him he wanted to withhold hundreds of millions of dollars in military aid from Ukraine until it helped him with investigations into political foe Joe Biden. Trump said Monday he never told Bolton such a thing.

A line of defense in Trump's Senate impeachment trial may sound very similar to one in the first impeachment case in American history. Back in 1868, a lawyer for President Andrew Johnson argued that Johnson couldn't be removed from office because Johnson hadn't committed a crime.

Today, one of Trump's lawyers, Alan Dershowitz, offered that very argument at Trump's trial — that impeachment requires criminal-like conduct.

But legal scholars dispute the idea that the Founding Fathers ever intended for impeachable offenses to require proof of a crime. Historians also are skeptical about crediting that argument with securing Johnson's narrow acquittal.

The articles of impeachment charge Trump with abuse of power and obstruction of Congress. The decision on witnesses may be complicated by reports Trump said he wanted to maintain a freeze on military assistance to Ukraine until it aided political investigations into his Democratic rivals. That's from former national security adviser John Bolton in a draft of his forthcoming book. 

Bolton writes in the book that Trump told him that he wanted to withhold security aid from Ukraine until it helped him with politically charged investigations, including into Democratic rival Joe Biden. Trump's legal team has repeatedly insisted that the Republican president never tied the suspension of military assistance to investigations into Biden and his son.

Read More: What To Watch For As Trump's Legal Team Resumes Its Impeachment Defense (via