A phone call between President Donald Trump and Ukraine's new leader, Volodymyr Zelensky, in which Trump is believed to have pushed Zelensky to investigate the Democratic presidential candidate Joe Biden's son, has been building into a political firestorm.

Now, Sen. Mitt Romney of Utah is wading in.

"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," Romney tweeted on Sunday. "Critical for the facts to come out."

With those words, Romney, the 2012 Republican presidential nominee, became the first major GOP politician to express concerns about the Ukraine call. Democrats, including Biden, have demanded that a transcript of the call be released. The House Intelligence Committee's chairman, Rep. Adam Schiff of California, said Sunday that if reports were true, impeachment "may be the only remedy."

Read more: A mysterious exchange between Trump and a foreign leader is Washington's latest obsession. Here's what is actually going on.

The call in question came under scrutiny after a member of the intelligence community filed a whistleblower complaint last month. The Washington Post reported last week that the complaint centered on Ukraine and a "promise" Trump made while in contact with a foreign leader. The July 25 call, in which Trump has acknowledged discussing Biden and Biden's son with Ukraine's president, is believed to be at the center of the complaint, though it has not been confirmed.

Trump has denied that anything improper occurred during the call.

Both Trump and his personal lawyer, Rudy Giuliani, have linked Biden's push while vice president for the ouster of Ukraine's top prosecutor in 2016 to the prosecutor's investigation of the owner of a company on whose board Biden's son Hunter served. Biden has denied coordinating with his son, and multiple news outlets have cast doubt on Trump's narrative.