GOP presidential candidate Bill Weld suggested on MSNBC's "Morning Joe" Monday that President Trump should face the death penalty for treason if he pressured Ukrainian President Volodymyr Zelensky to investigate Joe Biden and his son or face the withholding of military aid.

Weld's comments:

"[Trump] has now acknowledged that in a single phone call right after he suspended $250 million of military aid to Ukraine, he called up the president of Ukraine and pressed him 8 times to investigate Joe Biden, who the president thinks is going to be running against him."

"Talk about pressuring a foreign country to interfere with and control a U.S. election. It couldn't be clearer. And that's not just undermining democratic institutions. That is treason. It's treason, pure and simple. And the penalty for treason under the U.S. Code is death. That's the only penalty. The penalty on the Constitution is removal from office. And that might look like a pretty good alternative to the president if he can work out a plea deal."

The big picture: It's still unclear exactly what Trump told Zelensky, and the president said yesterday that "it doesn't matter" what he said.

Democrats across government have demanded that Trump release the transcript of the phone call to prove he has nothing to hide.

The incident has renewed calls for impeachment from Democrats — even more moderate members who had previously expressed reticence, like House Intelligence Chairman Adam Schiff.

Sen. Mitt Romney, a Republican who has been critical of Trump in the past, called the allegations surrounding the call as "troubling in the extreme."

The bottom line: Trump leads Weld by more than 80 percentage points in most recent polls for the GOP nomination in 2020, per Real Clear Politics.

Go deeper: Pelosi hints at impeachment over Trump-Ukraine whistleblower complaint