The Republican presidential candidate Bill Weld on Monday accused President Donald Trump of treason amid reports that Trump repeatedly urged the Ukrainian president to investigate former Vice President Joe Biden and his son.

Weld said death was "the only penalty" for treason.

The former Massachusetts governor made the astonishing remark during a joint interview on MSNBC's "Morning Joe" alongside Joe Walsh and Mark Sanford, former congressmen who are also mounting long-shot presidential bids to wrest the GOP nomination from Trump.

"That's not just undermining democratic institutions. That is treason. It's treason, pure and simple," Weld said. "And the penalty for treason under the US Code is death. That's the only penalty."

Business Insider's GOP presidential debate is Tuesday from 7 to 8:30 p.m. ET and will be exclusively livestreamed on "Business Insider Today's" Facebook Watch page.

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The Republican presidential candidate Bill Weld accused President Donald Trump of treason amid reports that Trump repeatedly urged the Ukrainian president to investigate former Vice President Joe Biden and his son — and Weld said death was "the only penalty."

Weld made the astonishing remarks during a joint interview on MSNBC's "Morning Joe" on Monday alongside Joe Walsh and Mark Sanford, former congressmen who are also mounting long-shot presidential bids to wrest the GOP nomination from Trump.

"That's not just undermining democratic institutions. That is treason. It's treason, pure and simple," Weld said. "And the penalty for treason under the US Code is death. That's the only penalty."

The former Massachusetts governor went on: "The penalty under the Constitution is removal from office, and that might look like a pretty good alternative for the president if he could work out a plea deal."

He doubled down on his call to impeach Trump, whom he described as "a lawless man" who had met the Constitution's threshold of "high crimes and misdemeanors" to remove an official from office.

Read more: Trump's rivals for the 2020 Republican nomination are at a huge disadvantage when it comes to name recognition

Weld and Walsh are set to face off in a livestreamed GOP primary debate organized by Business Insider on Tuesday. Sanford declined the invitation to attend.

Trump has been engulfed in a firestorm since The Wall Street Journal reported on Friday that he urged the Ukrainian president to investigate Joe and Hunter Biden over corruption in Ukraine several years ago.

After initially denying it, Trump acknowledged over the weekend that he had discussed the Bidens, as his conservative allies defended him.

Ukrainian officials have said there is no evidence of wrongdoing by the Bidens.

Business Insider's GOP presidential debate is Tuesday from 7 to 8:30 p.m. ET and will be exclusively livestreamed on "Business Insider Today's" Facebook Watch page.