According to his lawyer, he was “personally involved” in Mr Trump’s dealings with Ukraine officials.
If Mr Bolton were to testify, Democrats believe he could provide the link which proves Mr Trump withheld hundreds of millions of dollars of aid to the Ukraine as a bargaining chip to push the Ukrainians into investigating his rival for the 2020 election, Democrat Joe Biden.
Evelyn Farkas, who served as top Russia official during the Obama administration and is now running as a Democrat for a congressional seat in New York, said: “Bolton was present during the crime. And he knew that it was a crime at the time”
The New York Times has now cited a leaked manuscript from Mr Bolton’s book which states Mr Trump told him he wanted to freeze the aid until Ukraine helped with investigations.