(CNN) The top US diplomat in Ukraine expressed serious misgivings about foreign policy moves being tied to political motives, calling a potential quid pro quo over military assistance to Ukraine "crazy" and suggested he would quit if that assistance was not released, according to text messages released by the House Foreign Affairs, Intelligence and Oversight Committees on Thursday night.

Those texts -- given to the committees by former US Special Representative for Ukraine Kurt Volker as part of his deposition Thursday -- show Ambassador William "Bill" Taylor, the charge d'affaires at the US Embassy in Kiev, repeatedly questioning the decision to stall hundreds of millions of dollars in military aid to Ukraine as a potential quid pro quo and raising concerns about the impact on broader regional policy.

"The message to the Ukrainians (and Russians) we send with the decision on security assistance is key. With the hold, we have already shaken their faith in us. Thus my nightmare scenario," Taylor wrote on September 9. Russia has made repeated illegal incursions into Ukraine.

"As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign," he added.

The texts also show Volker linking Ukrainian President Volodymyr Zelensky's desire to meet with President Donald Trump with a bid for Ukraine to investigate the 2016 US election and Burisma -- the Ukrainian company that hired Hunter Biden to be on its board. The text messages do not explicitly mention the Bidens. Trump's private lawyer Rudy Giuliani was an impelling force behind the desire for such investigations.

Taylor's reservations about Ukrainian foreign policy being politicized were frequently answered by US Ambassador to the EU Gordon Sondland. The differences in responses between Taylor -- an experienced diplomat who served as US Ambassador to Ukraine from 2006 to 2009 -- and Sondland -- a former hotelier who donated to Trump's inauguration committee through four of his limited liability companies -- are notable. Taylor has been the top diplomat at the US Embassy in Kiev since June. He took on the role after Ambassador Marie Yovanovitch was abruptly recalled from her post in May. Sondland has been the ambassador to the European Union since late June 2018. He is scheduled to give a deposition to the committees next week. Sondland seemed to downplay the concerns raised by his counterpart in Kiev.

"Gordon, one thing Kurt and I talked about yesterday was Sasha Danyliuk's point that President Zelenskyy is sensitive about Ukraine being taken seriously, not merely as an instrument in Washington domestic, reelection politics," Taylor wrote on July 21.

Sondland replied, "Absolutely, but we need to get the conversation started and the relationship built, irrespective of the pretext."

On September 1, Taylor raised a question about the conditions upon which the aid was stalled and a White House visit by Zelensky would be predicated.

"Are we now saying that security assistance and WH meeting conditioned on investigations?" Taylor asked.

"Call me," Sondland replied.

On September 8 and 9, Taylor suggested the continued delay in security aid was benefiting Russia, referring to potential "nightmare" situations on both days. On September 8 he alluded to quitting in protest.

"The nightmare is they give the interview and don't get the security assistance. The Russians love it (And I quit.)," he wrote on September 8.

Sondland, in response to Taylor's suggestion on September 9 that the assistance was being withheld "for help with a political campaign," said that the diplomat was "incorrect about President Trump's intentions."

"The President has been crystal clear no quid pro quo's of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign," Sondland wrote. He suggested Taylor contact Secretary of State Mike Pompeo or his executive secretary to discuss it further.

Sondland's extensive involvement in Ukraine -- given that he is EU ambassador -- has raised questions. Sondland told UATV in July, "President Trump has not only honored me with the job of being the US ambassador to the EU, but he's also given me other special assignments, including Ukraine."

"We have what are called the three amigos, and the three amigos are Secretary Perry, Ambassador Volker and myself. And we've been tasked with sort of overseeing the Ukraine-US relationship between our contacts at the highest levels of the US government, and now the highest levels of the Ukrainian government," Sondland said in that interview.

On Friday, Trump sought to downplay his involvement in the ambassador's exchanges, saying, "I don't even know most of these ambassadors. I didn't even know their names."

However, the President immediately went on to praise Sondland for declaring there had not been any quid pro quo.

"The text message that I saw from Ambassador Sondland -- who's highly respected -- was: There's 'no quid pro quo.' He said that," Trump said.

Sondland, a political appointee, came to the diplomatic world from the business one. According to his State Department biography, he was "the Founder and CEO of Provenance Hotels, a national owner and operator of full-service boutique 'lifestyle' hotels. He is a longtime Republican donor.

Taylor has held a variety of diplomatic roles. In addition to earlier serving as ambassador to Ukraine, Taylor oversaw assistance and support to Egypt, Tunisia, Libya and Syria at the State Department during the Arab Spring, "served in Jerusalem as the U.S. Government's representative to the Mideast Quartet," "oversaw reconstruction in Iraq from 2004 to 2005, and served in Kabul as coordinator of international and U.S. assistance to Afghanistan from 2002 to 2003," according to his State Department biography. He was most recently the Executive Vice President of the US Institute of Peace.

The US Embassy in Kiev referred questions about the texts to the State Department's Bureau of European and Eurasian Affairs, which did not immediately reply to requests for comment. Sondland's spokesperson referred inquiries to the White House, which also did not immediately reply.

CORRECTION: This article has been corrected to state that Gordon Sondland donated to Trump's inauguration committee through four of his limited liability companies.