President Trump attacked acting U.S. Ambassador to Ukraine Bill Taylor as a "Never Trumper" in a tweet Wednesday, after Taylor testified to House investigators that Trump had withheld military aid until Ukraine agreed to investigate his political opponents.

Never Trumper Republican John Bellinger, represents Never Trumper Diplomat Bill Taylor (who I don’t know), in testimony before Congress! Do Nothing Democrats allow Republicans Zero Representation, Zero due process, and Zero Transparency. Does anybody think this is fair? Even though there was no quid pro quo, I’m sure they would like to try. Worse than the Dems!

The big picture: Taylor is a long-standing public servant who graduated from West Point, fought in Vietnam, and has worked at the Department of Energy, the Senate, NATO, the State Department and the United States Institute of Peace. He has served in every administration since 1985 and was asked to return to serve as acting ambassador to Ukraine in May by Secretary of State Mike Pompeo.

In his testimony, Taylor expressed concerns that while there is an official channel for U.S.-Ukraine policy decisions, there has also been also an "irregular, informal" back channel run by Rudy Giuliani, special envoy to Ukraine Kurt Volker, Energy Secretary Rick Perry and EU Ambassador Gordon Sondland.

Taylor testified that Trump withheld desperately needed military aid as leverage to get Ukraine to investigate Joe Biden and allegations of interference in the 2016 election on behalf of Democrats — the quid pro quo that Trump and his allies have long denied.

Taylor is being represented by attorney John Bellinger, who Trump also referred to as a "Never Trumper."

Earlier Wednesday, Trump tweeted that "Never Trumper Republicans, though on respirators with not many left, are in certain ways worse and more dangerous for our Country than the Do Nothing Democrats. Watch out for them, they are human scum!"

Go deeper ... Reports: Ukraine felt early Trump pressure and knew of military aid freeze