Former President Donald Trump has launched a scathing personal attack on fellow Republican Mitch McConnell.
"Mitch is a dour, sullen, and unsmiling political hack," said Mr Trump, "and if Republican Senators are going to stay with him, they will not win again."
His statement came after Mr McConnell said the former president was "morally responsible" for the US Capitol riots.
The exchange exposed an apparent rift between the two over the future of the Republican party.
The majority of Republican senators backed Mr Trump in his impeachment trial last week, including Mr McConnell who voted to acquit.
But in a speech on the Senate floor, Mr McConnell implied the former president should face criminal and civil litigation because he was "still liable for everything he did while in office".
He said Mr Trump's actions prior to the riots on 6 January amounted to "a disgraceful dereliction of duty".
The feud has prompted concern among some Republicans about the impact it may have on next year's mid-term elections. The party is trying to regroup having lost the House, the Senate and the presidency.
"I'm more worried about 2022 than I've ever been," Senator Lindsey Graham told Fox News.
"I don't want to eat our own," he added. "We need to knock this off."
What did Trump say
Mr Trump responded on Tuesday with his lengthiest statement since he left office a month ago.
"The Republican Party can never again be respected or strong with political 'leaders' like Senator Mitch McConnell at its helm," the press release reads.
Mr McConnell's "lack of political insight, wisdom, skill, and personality" cost the Republicans control of the Senate following last November's elections, he said.
Two closely-contested run-off votes in Georgia saw the Republicans lose both seats to Democrats, resulting in a 50-50 split of seats in the Senate. This leaves the Democrats in control because Vice-President Kamala Harris has the casting vote in the case of a tie.
Mr Trump also took aim at Mr McConnell's stated intentions to stand in the way of future Trump-backed candidates.
The former president said he would back Republican primary challengers who "espouse Making America Great Again and our policy of America First".
Mr McConnell "will never do what needs to be done, or what is right for our Country," said Mr Trump.
Politico reported that an early draft was even more personal, with a source telling the site it said Mr McConnell had "too many chins but not enough smarts".