Transcript for Top US diplomat to testify in impeachment inquiry
We begin with the impeachment showdown. Crucial witnesses headed to capitol hill. The ambassador who warned fellow diplomats it would be crazy to hold back aid to Ukraine and this comes as president trump is pressuring Republicans calling on them to get tougher and fight. Our senior congressional correspondent Mary Bruce starts us off. Reporter: Good morning, George. Well, president trump is now staring down the very real possibility of impeachment in the house and now he is trying to rally his own party telling Republicans this morning, it's time to toughen up. As the impeachment pressure grows this morning president trump is pushing Republicans to fall in line. I think the Republicans should get tougher. Reporter: Overnight the president taking on detractors in his own party. I think that the Democrats are not good politicians, but you don't have the Mitt Romneys of the world. You don't have people that will go against the party. You have better -- they stick together. Reporter: Senator Romney one of a few criticizing the president signalling he's open to removing trump from office. We can't have presidents asking foreign countries to provide something of political value that is after all against the law. Reporter: This morning on capitol hill another key witness, bill Taylor, the top U.S. Diplomat in Ukraine, he raised concerns when he thought the president was withholding millions of dollars in military aid for political gain. In released text messages between Taylor and the U.S. Ambassador to the eu, Gordon sondland, Taylor write, I think it's crazy to withhold security assistance for help with a political campaign. Trump is brushing it off. They're interviewing ambassadors who I had never heard of. I don't know who these people Reporter: But ambassador sondland was handpicked by trump for the job. Now, speaker Pelosi is also trying to rally her troops. She's circulated a fact sheet breaking down their messaging. What they've learned so far it describes what the speaker describes as the shakedown, the pressure campaign and the cover-up, George. Democrats are clearly trying to hammer away at their belief the president abused his office for political gain. Meantime, you've got the steady stream of witnesses behind closed doors for now and have so many new leads it's probably slowing the time line for impeachment down. Reporter: Yeah, George, Democrats say they want to move quickly here but also want to question a lot of these witnesses. They are plowing through these closed door depositions but know they still have open hearings that they want to hold so it seems likely this process is going to go well into the holiday season, speaker Pelosi told me last week the time line here depends on the truth line, George. Mary Bruce, thanks very much.
This transcript has been automatically generated and may not be 100% accurate.