Bill Taylor, a top US diplomat to Ukraine. Ukrinform/Barcroft/Getty Images

House Democrats have requested a deposition from Bill Taylor, the top US diplomat in Ukraine, multiple sources familiar with the issue told CNN.

However his interview has not yet been formally scheduled, the source said.

The request is expected to be met with opposition from the White House and State Department, and could present a quandary for Taylor, according to former State Department officials who know him.

Those officials suggested that if Taylor was made to choose between staying on to guide US policy as charge d'affaires in Kiev -- a post he came out of retirement to take -- or resigning to be able to testify, he would choose the former.

What's his role in all this? Taylor was thrust into the public eye following the release of his text exchanges with Kurt Volker, former special envoy for Ukraine, and Gordon Sondland, Trump's appointee for US ambassador to the European Union,

In the texts, Taylor repeatedly questioned the decision to stall hundreds of millions of dollars in military aid to Ukraine as a potential quid pro quo, calling it "crazy."

Will he actually testify? It appears unlikely. On Tuesday, just hours before he was due to appear, the State Department instructed Sondland not to testify at his scheduled hearing. Of the five diplomats scheduled to testify by the House, only Volker has been deposed. He resigned from the State Department days before his congressional appearance in order to testify -- an option that Taylor could take.