Mr Trump has been blamed for his party's worse-than-expected showing on November 8 after he endorsed hundreds of extremist candidates who proved unpalatable.
They included many "election deniers" who were pushing his false claims that an elaborate, nationwide conspiracy was to blame for his defeat to Joe Biden in 2020.
"It's Mitch McConnell's fault," Mr Trump wrote on his Truth Social platform, which is very similar to Twitter but with far fewer users.
He said "everyone despises" the senior Republican senator and used what Chinese Americans consider a racist slur against his Taiwan-born wife, Elaine Chao.
Mr Trump and said the Republican Party should have spent more money backing Jake Masters, who lost in his bid for a Senate seat in Arizona.
Democrats kept control of the US Senate on Saturday, repelling Republican efforts to retake the chamber and making it harder for them to thwart Mr Biden’s agenda.
Mr Trump is still considered the leader of the Republican Party but after last week's lacklustre electoral showing, in which the Democrats still have an outside chance of winning the House of Representatives, there is a palpable shift away from him.
Senior Trump allies are urging him to postpone his expected launch of a 2024 presidential run on Tuesday and conservative media outlets are rallying around Florida Governor Ron DeSantis as the future of the Republican Party after he won his bid for re-election.
Maryland's departing Republican Governor Larry Hogan said on Sunday that the party had to switch gears.
"It's basically the third election in a row that Donald Trump has cost us the race, and it's like three strikes and you're out," Mr Hogan, a vocal Trump critic, on Sunday told CNN.
"The definition of insanity is doing the same thing over and over again, expecting a different result.
"Donald Trump kept saying, 'we're gonna be winning so much, we'll get tired of winning'. Well, I'm tired of losing. I mean, that's all he's done."