Texts released by Democrats in Congress reveal how US officials worked to push the Ukrainian president into opening a public inquiry into President Trump's leading opponent, Joseph Biden.
The messages come as Mr Trump faces a formal impeachment inquiry about it.
The exchanges show a senior diplomat saying it would be "crazy" to withhold military aid to Ukraine for Mr Trump's political gain in the 2020 election.
US law bans soliciting foreign help for electoral purposes.
But Mr Trump denies any wrongdoing and has publicly reiterated his plea to Ukraine and China to investigate Mr Biden and his son Hunter over their political and business dealings, even though he has not offered specific evidence.
He accuses his opponents of a "witch hunt".
Impeachment in the House of Representatives would remove Mr Trump from office - but only if endorsed by the Republican-controlled Senate.
The impeachment inquiry stems from a 25 July call in which Mr Trump asked President Volodymyr Zelensky to investigate his rival.
The wide-ranging text message discussions are connected to allegations by the Democrats that Mr Trump dangled military aid as a way to press Ukraine to dig dirt on Mr Biden.
Sent between July and September, they involve high-ranking officials:
On 19 July, the three diplomats discussed arranging a phone call between Mr Zelensky and Mr Trump.
Mr Volker told the other two he had had breakfast with Mr Trump's lawyer, Rudy Giuliani that morning - and identified the main purpose of the upcoming phone call.
"Most impt [important] is for Zelensky to say that he will help the investigation," he told the others.