Former Massachusetts governor and longshot candidate for the GOP nomination for president Bill Weld on Monday accused President Donald Trump of "treason" for pressing Ukraine's President to investigate former Vice President Joe Biden and stated bluntly "the penalty for treason under the US code is death."

"Talk about pressuring a foreign country to interfere with and control a US election, it couldn't be clearer," Weld said in an interview on MSNBC's "Morning Joe." "And that's not just undermining democratic institutions, that is treason. It's treason pure and simple."

"The penalty for treason under the US code is death. That's the only penalty," Weld added. "The penalty under the Constitution is removal from office and that might look like a pretty good alternative to the President if he could work out a plea deal."

Weld's comments come amid mounting scrutiny of Trump after he acknowledged discussing Biden and his son, Hunter, in a July call with Ukrainian President Volodymyr Zelensky. The call was part of a whistleblower complaint submitted to the Intelligence Community Inspector General, another person familiar with the situation told CNN.

White House officials are now considering releasing a transcript of the call, multiple sources have told CNN. There is no evidence of wrongdoing by either Joe or Hunter Biden.

Weld made the comments in a joint interview alongside two other Republicans challenging Trump for the GOP nomination, former congressmen Joe Walsh and Mark Sanford.

"Donald Trump needs to be impeached. Period," Walsh said. "As Bill Weld just said, he told a foreign leader two months ago to interfere in our 2020 election. He needs to be impeached."

Sanford, who also served as governor of South Carolina, echoed Weld and Walsh's strong rhetoric.

"I think you know where the three of us are on this larger debate," he said.

Earlier this month, all three GOP challengers penned an op-ed in The Washington Post lambasting state Republican leaders and Trump for opting to cancel their 2020 presidential primary elections in a show of support for the President.

"The primary nomination process is the only opportunity for Republicans to have a voice in deciding who will represent our party," they said. "Let those voices be heard."

The canceled primaries underscore the difficulty the long-shot candidates will have in challenging Trump, who, according to a CNN/SSRS Poll this month, has an 88% approval rating among Republicans.