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Bill Taylor has become famous, at least to those following the impeachment inquiry, for his text messages. The acting head of the U.S. Embassy in Ukraine was the one who told other U.S. diplomats, quote, "I think it's crazy to withhold security assistance for help with a political campaign." Today Taylor told the House Intelligence Committee he believed that then, and he still believes it. NPR's Claudia Grisales has seen Bill Taylor's written testimony. She joins us now with details.

Claudia, what else did you learn?

CLAUDIA GRISALES, BYLINE: Well, we learned that Democrats who attended the deposition today told NPR that this testimony is some of the most significant to the impeachment inquiry yet. Taylor said he did not agree with what the Trump administration was doing. He raised concerns about it with then-National Security Adviser John Bolton. And he filed a cable about it with the secretary of State, Mike Pompeo. Taylor says he told people in the administration he was prepared to resign when military assistance was frozen for Ukraine because he believed the U.S. should support it strongly. And the bottom line here is that people at Taylor's level were seeing firsthand what we've also heard from others about this Ukraine story. The Trump administration wanted to hold up assistance for Ukraine, in engagement with Trump, until and unless it got a commitment from Ukraine's government that it would do the investigations that Trump wanted.

CORNISH: What has the White House said in defense?

GRISALES: Well, in a statement this evening, White House spokesperson Stephanie Grisham called it a, quote, "coordinated smear campaign from far-left lawmakers and radical unelected bureaucrats waging war on the Constitution." The White House denied that Trump officials made a quid pro quo to Ukraine that the country investigate the Biden family in exchange for U.S. military assistance, as Taylor alleges.

CORNISH: Put this in context. How does it advance our kind of overall understanding of this story so far?

GRISALES: So Bill Taylor was the guy in the room for a lot of this. He was dealing with the Ukrainians firsthand, so he was involved closely with all of these events taking place. Now, we don't know all of what Taylor said. We have his written statement, which was first reported by some of our colleagues in Washington today, but NPR's verified it. But Democrats call this evidence incredibly compelling today. And tonight, House Republicans took to the floor in a bit of a speech rally, if you will, to defend the president and, basically, to shoot down claims that this process is moving along in a fair way. They say the hearing is being held in a bunker in the capital behind closed doors, and so they showed a lot of solidarity there in terms of supporting the president.

CORNISH: That's NPR's Claudia Grisales.

Thanks so much.

GRISALES: Thanks for having me. Transcript provided by NPR, Copyright NPR.