House Democrats Thursday night released a trove of explosive text exchanges between top U.S. diplomats that provides a closer look into U.S. President Donald Trump's months-long effort to pressure Ukraine's leader to investigate former Vice President Joe Biden.

The text messages, provided to House committees by then special envoy to Ukraine Kurt Volker, show that the Trump administration attempted to use a possible meeting between the U.S. president and Ukrainian leader Volodymyr Zelensky to pressure Kyiv to launch an investigation into Biden and his son Hunter.

In which Volker informs Zelensky’s aid that a Zelensky visit to Washington is predicated on convincing Trump that he’ll “get to the bottom of what happened” in 2016. pic.twitter.com/mQk62rlOvk — Natasha Bertrand (@NatashaBertrand) October 4, 2019

The messages also showed Bill Taylor, the top American diplomat in Ukraine, raising alarm about Trump's attempt to withhold aid to Ukraine for electoral purposes.

"Are we now saying that security assistance and WH meeting are conditioned on investigations?" Taylor asked Volker and U.S. Ambassador to the European Union Gordon Sondland on Sept. 1, before the whistleblower complaint about Trump's call with Zelensky went public.

Sondland replied simply, "Call me."

Eight days later, Taylor wrote to Volker and Sondland, "As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign."

Sondland insisted that Trump "has been crystal clear no quid pro quo's of any kind."

"Sondland taking five hours to respond, talking to Trump, and then replying 'no quid pro quo' shows 1) they knew what they were doing 2) knew it was wrong 3) settled on the 'no quid pro quo' defense before it ever became public," wrote MSNBC's Chris Hayes.

Read the text messages:

Observers said the text messages thoroughly undermine Trump's claim that he was not seeking a quid pro quo with Ukraine.

"These Kurt Volker text messages are FILLED to the BRIM with quid pro quo," said Brookings Institution fellow Scott Anderson. "I never expected anything this explicit in writing. It's truly astounding."

Sen. Chris Murphy (D-Conn.) directed a tweet at Trump's Republican defenders.

"If you're a Republican who hung your hat on 'no quid pro quo!', what do you do tomorrow?" Murphy wrote. "The texts make 100 percent clear: 1. Our top diplomat in Kiev says there was an "investigation for aid" quid pro quo. 2. Everyone knew there was a 'investigation for meeting' quid pro quo."