And so the reaction from Tory backbenchers over the next few days - to finishing behind the anti-European party UKIP - will be critical.
How to respond to an election result in which your party finished third - in a seat you have to win in a general election in order to get a majority in the Commons.
Lurch to the right? Restore those "core Tory values" to bring back traditional voters? Take a stronger line on Europe? Reverse plans for gay marriage and return to the promise to recognise marriage in the tax system? That approach will be called for by many in the party and its supporters/critics in the right wing press.
Their analysis of David Cameron's problems is that he's gone too liberal, that his chosen path for government has strayed from the Conservative route they had anticipated.
But one of the Prime Minister's close allies, Education Secretary Michael Gove, has already ruled out that approach this morning.
So how concerned should Conservative supporters be?
They can take heart from the fact that Eastleigh was a by-election. And voters can chose a party knowing it will not change the Prime Minister of the day. A mid-term protest vote at a by- election is nothing new.
And with two parties in a coalition government - and Labour so recently in power - there was one obvious place to go - UKIP.
But had a spectacular result in Eastleigh - its best ever - and it came just 1,700 votes from getting its first MP (elected not defected) in the party's history. (I personally think they might have taken the seat had Nigel Farage stood here instead of its candidate Diane James).
With the European elections next year - that presents a big problem for David Cameron.
Voters did warm to the Prime Minister's promise of a referendum and a fight with Europe - but they warmed more to Nigel Farage's promise to leave Europe immediately and to cut immigration (although ironically there was a dramatic fall in immigration announced on the day of the by election).
And if UKIP can maintain the momentum - it risks splitting the vote on the right. That may not win them any seats in a first past the post contest at the general election but it will drain enough support from the Conservatives to hand a number of seats to Labour or the Lib Dems.
The general election in 2015 is still more than two years away. Still time for David Cameron to turn the economy round (which will ultimately decide the winner of the next election).
But Chancellor George Osborne is now under greater pressure in the budget this month to show voters and his backbenchers he is listening to their concerns.
Oh and the Lib Dems? What an escape/triumph. We're talking about David Cameron's woes today. Not Nick Clegg's.