Bill Taylor, the current US Ambassador to Ukraine, is set to appear before the House Intelligence, Oversight, and Foreign Affairs Committees on Tuesday. Taylor is likely to be asked about his concerns about a possible Trump-Ukraine quid pro quo. "I think it's crazy to withhold security assistance for help with a political campaign," Taylor wrote in a September text message exchange. Leigh Ann Caldwell explains why Taylor’s deposition could be a lynchpin for House investigators. Our guest host, Carrie Dann, fills in for Steve Kornacki.

Questions about the impeachment inquiry? Email us at [email protected].

For transcripts and further reading, check out https://www.nbcnews.com/articletwo.

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