Keir Starmer and Rishi Sunak both appeared to take swipes at Boris Johnson's Brexit blunders as the former PM's legacy lies in tatters.
Mr Starmer said Labour would put "country before party" and back the Windsor Framework - accusing Mr Johnson of talking "nonsense" over the botched EU deal it replaces.
And the current Prime Minister - who was a member of the cabinet when the agreement was reached - seemed to concur, telling the House of Commons: "I agree with him on the substance of what he said."
He then called on MPs "not to look backwards, but to look forward".
Mr Starmer told MPs that Labour would not "snipe" or "play political games". But he urged the latest Prime Minister to repay that faith by being "utterly unlike his predecessor".
He said that Mr Johnson's 2020 agreement had led to the collapse of power-sharing in Northern Ireland, leaving the country without an effective devolved government.
There were shouts of "where is he?" from Labour benches, as the ex-PM was nowhere to be seen in the Commons.
Referring to Mr Johnson, Mr Starmer said: "The Member for Uxbridge and South Ruislip (Mr Johnson) told the people of Northern Ireland that his protocol meant 'no forms, no checks, no barriers of any kind' on goods crossing the Irish Sea after Brexit.
"That was nonsense. A point-blank refusal to engage with unionists in Northern Ireland in good faith, never mind take their concerns seriously.
"And it inevitably contributed to the collapse of power-sharing in Northern Ireland."
Moving forward, he said, he hopes Mr Sunak will act with less "bluster" than Mr Johnson, stating: "So, when presenting what this agreement means in practice, I urge the Prime Minister to be utterly unlike his predecessor.
"Do not pretend the deal is something it is not. Where there are trade-offs to be made, argue the case for them.
"Treat unionists with the respect of frank honesty, not the contempt of bluster."
The Labour leader, who encouraged party members to "put country before party", admitted the deal is "not perfect", saying there will "inevitably be trade-offs".
But he rounded off by saying: "I hope that in the coming days others will come to support the agreement in the same spirit."
Mr Sunak told the Commons Mr Johnson's legislation kept the UK under the jurisdiction of the European Court of Justice, "leaving us open to months, maybe years, of uncertainty, disruption and legal challenge".
He said he believes "passionately with my head and my heart" that the Windsor Framework is the "right way forward".
He told the Commons: "We've achieved free-flowing trade with a green lane for goods, no burdensome customs bureaucracy, no routine checks on trade, no paperwork whatsoever for Northern Irish goods moving into Great Britain and no border in the Irish Sea.
"We've protected Northern Ireland's place in the Union with state aid reach-back fixed, the same tax rules applying everywhere, vet certificates for food lorries gone, the ban on British sausages gone, parcel paperwork gone, pet paperwork gone, garden centres now selling the same trees, supermarkets selling the same food, and pharmacies selling the same medicines.
"And we've safeguarded sovereignty for the people of Northern Ireland with the democratic deficit closed, the Vienna Convention confirmed, thousands of pages of EU law scrapped and with the Stormont brake we have safeguarded democracy and sovereignty for the people of Northern Ireland."
