Final day for Trump's defense after John Bolton bombshell

The White House along with the president's legal team has scrambled to contain the damage from former national security advisers that claimed Trump told him Ukraine aid was tied to investigating Joe Biden.
4:48 | 01/28/20

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Transcript for Final day for Trump's defense after John Bolton bombshell
We turn to Washington and the impeachment trial of president trump. Today is the final day for trump's legal team to make their case as the white house scrambles to contain the damage from the John Bolton bombshell. It has fueled new calls to call him as a witness in the trial. Mary Bruce is tracking all the latest on capitol hill. Good morning, Mary. Reporter: Good morning, George. Well, Republican leaders are trying to stay the course and the president's legal team insists these allegations don't change a thing but what is changing here this morning is the calculus of some key Republicans who now see increasingly likely to support calling John Bolton as a witness. Making their closing arguments the president's legal team is doubling down insisting there is no evidence the president explicitly tied military aid to Ukraine to investigations into his rivals. There was no connection between security assistance and investigations. Reporter: Making that claim even after explosive allegations from a firsthand witness, trump's former national security adviser John Bolton. He reportedly writes in his upcoming book the president told him directly he was holding nearly $400 million in aid from Ukraine until they agreed to investigate. Overnight trump's lawyer Alan Dershowitz arguing even if true, it's not impeachable. Nothing in the Bolton revelations, even if true, would rise to the level of an abuse of power or an impeachable offense. That is clear from the history. That is clear from the language of the constitution. Reporter: But pressure is building and key Republicans suggest they want to hear from Bolton. I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton. I've said before I was curious about what ambassador Bolton might have to say. I'm still curious. Reporter: Democrats say it's a game changer. Obviously the revelations in "The New York Times" about the Bolton boom strengthen the case for witnesses, for documents, even more. Reporter: Trump denies the allegations and is accusing Bolton of trying to boost the sale of his book. I haven't seen a manuscript but I can tell you nothing was ever said to John Bolton. Reporter: With Bolton ready to testify the president's team is going on offense arguing trump was justified in wanting to investigate Joe Biden and his son hunter's work for the Ukrainian gas company burisma. They said there was nothing, knock to see. It was a sham. In fact, every witness who was asked about hunter Biden's involvement with burisma agreed there was a potential appearance of a conflict of interest. Reporter: Biden maintains there is nothing to investigate. No one said I did anything that was wrong, period. Reporter: All eyes here are on four key Republican, Mitt Romney, Susan Collins, Lisa murkowski and Lamar Alexander but there is a very real sense that the dynamic here is shifting, that these witnesses are increasingly a very real possibility. That's to be from the senate. Let's bring in Jon Karl. We just heard Mary reporting on capitol hill they feel like the pressure is growing but the white house is really scrambling yesterday but at the end of the day they're more confident they can keep Bolton from testifying. Reporter: They are. It is a fluid situation. The white house 24 hours ago was bracing for the possibility of witnesses, Bolton and perhaps others but are increasingly confident this morning they can hold back the vote on this, that they can prevent the witnesses from being called. They're basically arguing that this would prolong the trial considerably because there would be legal challenges. That's the case they're making to those four moderates mentioned by Mary Bruce and also making the point even if John Bolton is called to testify, even if he says everything that was reported in his book, it will not change the final vote on removing Donald Trump from office. Basically the point that Alan Dershowitz made at the end of his presentation. Meantime, Jon, the revelations from this keep coming. "The New York Times" is reporting that Bolton in the manuscript talks about talking to the attorney general William Barr about his concerns about the president's dealings with dictators like XI and erdogan. Reporter: John Bolton was the national security adviser for 17 months. He was present for virtually all of the president's meetings with foreign leaders and with his top national security advisers. He is somebody who was a legendary note taker. There is likely to be much in this book beyond the revelations of Ukraine. You and Mary will join us for the live coverage. I'll anchor with our team starting at 1:00 eastern.

This transcript has been automatically generated and may not be 100% accurate.

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