Mike Turner, a Republican who sits on the House Intelligence Committee, was asked about testimony which alleges Mr Trump directed Gordon Sondland, the US ambassador to the EU, to pressure Ukraine into announcing an investigation into Joe Biden and his son Hunter.
A former National Security Council official told Congress he heard from Mr Sondland that Ukraine had to make an announcement about the investigation to receive US military aid.
“Of course, all of that is alarming. As I've said from the beginning, I think this is not OK,” Mr Turner told CNN’s Jake Tapper.
“The president of the United States shouldn't even in the original phone call be on the phone with the president of another country and raise his political opponent. So, no, this is not OK.”
Despite his concerns, the congressman went on to point to Mr Sondland’s testimony, in which the ambassador claimed the direction was not a “quid pro quo” by the president.
He also defended Mr Trump’s tweets attacking Marie Yovanovitch, the former ambassador to Ukraine, as she testified publicly.
Ms Yovanovitch said the president’s tweets were “very intimidating”, while Mr Turner described the tweets as "unfortunate" but not "witness intimidation".
However, Mr Turner’s answer indicates disquiet in the Republican Party about the allegations against Mr Trump.
The congressman said he could not comment on the closed-door testimony of David Holmes, a State Department official based in Kiev, which reportedly details a July call between Mr Trump and Mr Sondland.
Mr Holmes testified that he overheard Mr Trump asking the ambassador if Volodymyr Zelensky, the Ukrainian president, would “do the investigation”.
The official also told Congress that Mr Sondland said the president only cares about “the big stuff” that benefits him in Ukraine, referring to an investigation into the Bidens.
There is no evidence of wrongdoing by either Joe Biden or his son in Ukraine.
For Mr Trump to be removed from office, the House of Representatives must impeach him and the Republican-led Senate must then vote to convict him.