David Cameron has been warned that a repeat of the Eastleigh embarrassment is possible at the next election.
A post-mortem by Conservatives of their defeat in the by-election got underway within hours of the result.
One grassroots campaigner said the organisation had fallen into a state of disrepair at ground level, meaning it was incapable of winning enough crucial seats from the Liberal Democrats at the 2015 general election.
Party grandee Lord Tebbit accused the Prime Minister of treating the party's grassroots "with contempt."
Writing in the Telegraph, Tebbit said: "The Tory grassroots shrivelled in the climate of contempt in which they are regarded by the Tory leadership."
Elsewhere, former leadership challenger David Davis warned the Prime Minister from the backbenches that the defeat amounts to a crisis for the Tories.
Ukip inflicted a huge upset on the Tory party by claiming second place in the poll. The result made Ukip leader Nigel Farage a bogeyman figure for Tory hopes of re-election.
Leading Tory euro-sceptic Douglas Carswell said Eastleigh suggested that Cameron's party was "on the long road to defeat."
Carswell openly questioned the direction the party was headed in, telling the BBC: "We talk about only a protest vote, [but] one of the reasons why people feel inclined to protest is because they are hurting in their pocket.
"What is it that's happened that's allowed us to go on this long retreat, this long march of defeat?"
The broadside from inside the party came as senior Tories flocked to protect the Prime Minister from mounting criticism of his leadership.
Desperate bids were made to put a gloss on the result, in which the party slumped to third place behind Ukip after being hopeful of victory before polling day.
Deputy party chairman Sarah Newton MP tried to paint the humiliating defeat as a win for coalition government.
She told the Western Morning News: "It is not a disaster for the Conservative Party.
"It was a Liberal Democrat seat to hold. They are part of the coalition. It was a win for the coalition.
"Most people thought the Liberal Democrats would win."
Yet Liberal Democrat leader Nick Clegg was not so eager to share his party's victory when he spoke after the result.
Clegg goaded the PM that he had to "explain, or seek to explain" why candidate Maria Hutchings polled 1,000 fewer votes than Ukip candidate Diane James.
Euro-phobic Ukip stormed the polls, scoring a sensational second place as part of a 19.3 percent swing away from the Lib Dems. Liberal Democrat Mike Thornton won the seat.