Text messages 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, Joe 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, on Thursday, publicly called on 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 of misconduct.
He accuses his opponents of a “witch hunt”.
Impeachment in the US House of Representatives would lead to a trial in the Republican-controlled Senate.
Senators would have to vote to convict Mr Trump by a two-thirds majority to remove him from office.