"What has been alleged by some news sources is that the president asked for an investigation into Mr. Biden. That would be in my view very inappropriate and a very serious allegation itself, so let's find out what exactly what was said," Romney added on Monday.

Romney was one of the first Senate Republicans to raise concerns about the whistleblower complaint. He said in a tweet over the weekend that it would be "troubling in the extreme" if Trump pressured a foreign leader to investigate a political opponent.

"If the President asked or pressured Ukraine’s president to investigate his political rival, either directly or through his personal attorney, it would be troubling in the extreme. Critical for the facts to come out," he tweeted.