Dorries announced on Saturday she was finally actually resigning as an MP, more than two months after she first said she would do so.
The former culture secretary and close ally of Boris Johnson launched a stinging attack on Sunak, warning the party faced an “electoral tsunami”.
Johnny Mercer, the veterans minister, on Sunday defended the prime minister from the “personal attack”.
“I don’t think we are on track to lose that by-election. I think we’ve got a good candidate up there,” he told Times Radio.
“I think people recognise that committing to these five priorities by the PM was always going to be difficult.
“We are just over half-way through the year. He has made this commitment over the year.
Mercer added: “It’s far better to be seen fail while striving greatly rather than chucking rocks from the side and looking at the situaiton thinking there is not much we can do.”
The prime minister has vowed to halve inflation, cut NHS waiting lists, grow the economy, reduce the national debt and stop the boats carrying asylum seekers across the English Channel.
Dorries dramatically announced in June she was quitting parliament “immediately” after she was denied a seat in the House of Lords.
But she delayed her formal resignation, preventing a by-election in her Mid Bedfordshire constituency from taking place before the summer.
Dorries came under increasing pressure to stand down from Tory MPs, frustrated at the fact she had remained in place despite not having spoken in the Commons chamber since June 7, 2022.
Sunak had said Dorries’ constituents were not being “properly represented”, while other Conservatives lined up to tell her to go.
Her decision to throw in the towel now means Rishi Sunak will return from parliament’s recess facing another potential damaging loss at the polls.
Dorries holds a healthy majority of 24,664 in Mid Bedfordshire, a lead that would usually be considered extremely safe. But both Labour and the Lib Dems have recently overturned similarly large Tory majorities.
In her resignation statement, Dorries attacked Sunak’s leadership of the party. “What exactly has been done or have you achieved? You hold the office of prime minister unelected, without a single vote, not even from your own MPs,” she said.
“You have no mandate from the people, and the government is adrift. You have squandered the goodwill of the nation, for what?”
Dorries added: “Your actions have left some 200 or more of my MP colleagues to face an electoral tsunami and the loss of their livelihoods, because in your impatience to become prime minister you put your personal ambition above the stability of the country and our economy.
“Bewildered, we look in vain for the grand political vision for the people of this great country to hold on to, that would make all this disruption and subsequent inertia worthwhile, and we find absolutely nothing.”
Dorries’ decision to quit came ahead of a move by the Lib Dems to stage a Commons vote on a new Bill that could have forced a by-election in her seat.