"Mitch is a dour, sullen, and unsmiling political hack, and if Republican Senators are going to stay with him, they will not win again."
Trump regrets support
The attack came after Mr McConnell said on Saturday that even though he had voted to acquit Mr Trump at his impeachment trial, the former president was nevertheless "practically and morally responsible" for the 6 January storming of the US Capitol.
After 50 Democratic senators and seven Republicans voted that Mr Trump was guilty - a majority in the 100-seat Senate, but not the two-thirds needed for a conviction – Mr McConnell let loose, excoriating Mr Trump for the attack that saw five people die and the halls of the US legislature ransacked by his supporters.
He called Mr Trump's actions leading up to the siege, including a White House rally where the then-president urged followers to descend on Congress, "a disgraceful dereliction of duty."
"These criminals were carrying his banners. Hanging his flags. And screaming their loyalty to him," Mr McConnell said.
But Mr Trump blamed the wily veteran lawmaker, who for four years as majority leader kept the Senate aligned with the Republican president, for losing party control of the body.
He claimed credit for Mr McConnell's own win of another six-year term representing Kentucky in the Senate, where the 78-year-old has served since 1984.
"My only regret is that McConnell 'begged' for my strong support and endorsement before the great people of Kentucky in the 2020 election, and I gave it to him," Mr Trump said.
"Without my endorsement, McConnell would have lost, and lost badly."
Mr Trump further blamed Mr McConnell for the party's loss of two Senate seats in a Georgia runoff election on 5 January, handing control of the Senate to Democrats.
Many analysts however blame that loss on Mr Trump himself, for his unsettling refusal to accept President Joe Biden's strong 3 November election victory, based on groundless claims of fraud.
It was that claim, both Mr McConnell and Democrats say, which incited the unprecedented attack on Congress on 6 January.
In another line of personal attack, Mr Trump took aim at Mr McConnell's wife, Taiwan-born Chinese-American Elaine Chow, who was transportation secretary in Mr Trump's cabinet but resigned in protest after the 6 January Capitol assault.
"McConnell has no credibility on China because of his family's substantial Chinese business holdings," Mr Trump wrote.
With his impeachment trial looming, the ex-president had been almost completely silent on politics since moving into his Mar-a-Lago country club in Palm Beach, Florida nearly one month ago.
But he used the blast against Mr McConnell to assert his continuing claim to leadership of the Republican party.
He threatened to use his continuing popularity among the Republican base to support any Republican candidates - the next national election is in November 2022 - who support his agenda.
"If Republican Senators are going to stay with him, they will not win again," he said.
"Where necessary and appropriate, I will back primary rivals who espouse Making America Great Again and our policy of America First."
"This is a big moment for our country, and we cannot let it pass by using third rate 'leaders' to dictate our future!" he added.
Meanwhile Mr McConnell came under criticism from Mr Biden for his position on the new administration's $1.9 trillion USD COVID-19 economic relief bill.
Mr McConnell and other Republicans have declined to endorse the Mr Biden package of stimulus measures, calling it too expensive.
Asked about Mr McConnell's reported comments that he could unite his party around opposition to the bill, Mr Biden said: "It may unify Republicans but it will hurt America badly."