The first public hearings are set to begin next week in the impeachment inquiry into President Trump.
Next Wednesday and Friday three U.S. diplomats will answer questions about President Trump's dealings with Ukraine.
Democrats hope the televised testimony will help their argument.
Acting ambassador to Ukraine, Bill Taylor, will testify on Wednesday. Deputy Assistant Secretary of State George Kent will also testify on Wednesday.
On Friday, Former Ambassador to Ukraine Marie Yovanovitch will appear for testimony. They have all previously given depositions behind closed doors.
At a rally in Louisiana Wednesday night, President Trump called the inquiry an illegal act.
Former National Security Advisor John Bolton is expected to ignore lawmakers' requests to testify on Thursday.