Washington (CNN) Republican presidential candidate Bill Weld called on President Donald Trump to resign Wednesday, writing in an op-ed that the country would be "better served with a President Mike Pence."

"If Donald Trump is an American patriot, he should resign from office," Weld, the only announced Republican primary challenger to the President, wrote in an op-ed published in The Bulwark, an online platform that publishes conservative commentary.

He added: "Trump's rampant dishonesty and paranoia render him incapable of serving as president. For once, he should put the good of the United States ahead of his own ego and resign."

Weld, the former Republican governor of Massachusetts, officially announced his primary challenge to Trump earlier this month, making him the President's only Republican opponent so far in the 2020 election. In 2016, Weld was the vice presidential nominee on the Libertarian Party ticket with former New Mexico Gov. Gary Johnson. He previously served two terms as the governor of Massachusetts in the early 1990s.

Weld is a staunch critic of the President, telling CNN's Jake Tapper on "The Lead" that it would be a "political tragedy" and he would "fear for the Republic" if the country had six more years of Trump as President. And, prodded by the release of a redacted version of special counsel Robert Mueller's investigation into Russian meddling in the 2016 election, Weld ratcheted up his rhetoric against the President, calling for his resignation.

"Time and again, Trump tried to use the power of the Oval Office to protect himself and his associates from the consequences of their actions. The only defense Trump has to obstruction of justice is that he was too incompetent to carry it off," Weld wrote.

Mueller's team investigated several specific instances of potential obstruction of justice in the report, but prosecutors ultimately did not make a decision on whether the President had obstructed justice. The report also did not exonerate him of criminal conduct

The Democratic Party has split on whether the special counsel's report warrants lawmakers beginning impeachment proceedings against the President, with some 2020 Democratic presidential candidates -- like Sens. Elizabeth Warren of Massachusetts and Kamala Harris of California and former Housing and Urban Development Secretary Julian Castro -- supporting the call while party leadership seems to have tamped down plans.

On impeachment, Weld wrote, "Whether this clear pattern of obstruction warrants impeachment is the purview of the House of Representatives."

He continued: "But regardless of what they decide, the facts revealed in the Mueller report confirm that Donald Trump is not to be trusted."