Former Massachusetts Gov. Bill Weld (R), who last month announced a committee to explore a GOP primary challenge to President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE, slammed the president Tuesday as a “national embarrassment” over his recent criticism of the late Sen. John McCain John Sidney McCainCindy McCain endorses Biden: He's only candidate 'who stands up for our values' Biden says Cindy McCain will endorse him Biden's six best bets in 2016 Trump states MORE (R-Ariz.).

“@realDonaldTrump's continued attacks on John McCain are contemptible and beneath the dignity of our American democracy,” Weld tweeted Tuesday. “While @realDonaldTrump's senseless attacks on allies have become all too common, his defamation of a deceased American hero has reached a new low and raises serious concerns about his mental stability and moral integrity.”

John McCain was an American hero. @realDonaldTrump is a national embarrassment. BW #Weld2020 (3/3) — Gov. Bill Weld (@GovBillWeld) March 19, 2019

The president turned to Twitter over the weekend to slam McCain over his 2017 vote to sink a Republican effort to repeal the Affordable Care Act and his alleged role in giving the FBI a dossier compiled by Christopher Steele detailing compromising material the Kremlin may have on Trump.

Spreading the fake and totally discredited Dossier “is unfortunately a very dark stain against John McCain.” Ken Starr, Former Independent Counsel. He had far worse “stains” than this, including thumbs down on repeal and replace after years of campaigning to repeal and replace! — Donald J. Trump (@realDonaldTrump) March 16, 2019

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“I was never a fan of John McCain, and I never will be,” Trump added Monday in the Oval Office when asked about his attacks on the Republican senator seven months after his death.

McCain, a former prisoner of war in Vietnam, 2008 GOP presidential nominee and Arizona lawmaker for over 30 years, was widely respected on both sides of the aisle.

Weld hammered Trump’s character during his presidential announcement in New Hampshire.

“[O]ur President is simply too unstable to carry out the duties of the highest executive office — which include the specific duty to take care that the laws be faithfully executed — in a competent and professional matter,” Weld said. “He is simply in the wrong place.”

“It upsets me that our energies as a society are being sapped by the president's culture of divisiveness,” he added.

Weld is angling his campaign’s appeal to “never-Trump” Republicans and independents who are irked by the president’s personal invectives against his perceived enemies.