The rise of the UK Independence Party is a major headache for David Cameron's Conservatives, but also has the potential to cause problems for the other two mainstream parties.
While Westminster's first-past-the-post system all but denies UKIP a chance of winning seats in the House of Commons, Mr Cameron is painfully aware that a split in the right-of-centre vote could rob him of victory at the 2015 general election.
Meanwhile, UKIP's strong showing in South Shields – following earlier second-place slots at by-elections in Middlesbrough and Rotherham – proved the party is capable of wooing "old Labour" voters in Ed Miliband's northern heartlands, as well as traditionalist Tories in the South. And Nick Clegg's Liberal Democrats are having to rethink their electoral strategy after seeing their long-standing position as the repository of protest votes comprehensively stolen by Nigel Farage's insurgents.
Mr Farage is insisting that UKIP should no longer be seen as a single-issue protest party, arguing that voters are attracted not only by their EU withdrawal message, but also by policies including low taxation and tight immigration controls. He said UKIP will use its "bridgeheads" in councils to build the kind of local operations it needs to challenge for Westminster seats. UKIP is setting its sights on first place in next year's European elections, as well as the possible bonus of a Westminster seat if a by-election comes up in a marginal constituency like Portsmouth South. And it will be hoping to keep the bandwagon rolling at least until 2015, when it could seize a few seats contested by high-profile candidates like Mr Farage.
There is little doubt that UKIP's success reflects mounting distrust in the major parties amongst an electorate battered by years of austerity, job losses, stagnating wages and cuts following the financial crisis.
However, it is far from certain that the voters who gleefully put the boot into governing parties in a mid-term council poll will do the same when the serious business of choosing a government comes round.
Its very success means that the UKIP policy platform and its slate of candidates will now come under the kind of intense scrutiny which has killed off many previous challengers to the political status quo. Mr Farage suggested that UKIP's goals may be achieved not by seizing power at Westminster but by forcing the Conservatives to adopt its agenda. He compared his party to the SDP, which broke away from Labour in the 1980s and paved the way for the eclipse of the Bennite left and the ascendancy of centrists under Tony Blair.
Certainly, the most immediate impact of Friday's election results will be to increase pressure on Mr Cameron to rethink his approach to core UKIP issues like Europe and the expected influx of Romanians and Bulgarians when work restrictions are lifted.
Internal party critics are arguing that the Prime Minister's EU referendum pledge and his tough rhetoric on restricting benefits for immigrants have not gone far enough to stem the tide of Tory defections.
Backbenchers were pressing him to table legislation during this Parliament to pave the way for a referendum on British withdrawal from the EU. Even though opposition from Liberal Democrat coalition partners means a Bill would almost certainly fail, right-wing Tories believe it would bolster voter confidence that Mr Cameron would deliver the post-election referendum, stealing a lot of UKIP's thunder in 2015.
The Prime Minister will also have to decide whether to stick to his guns on issues like gay marriage and protecting the overseas aid budget, which many of his MPs blame for turning off traditional Tories.
He knows that failure to attract back UKIP voters could transform dozens of safe Conservative seats into marginals vulnerable to Labour or the Lib Dems in the general election.
Labour, meanwhile, faces the danger that voters angered by the Government's record on the economy, jobs and public services might not take the usual route of switching to the main opposition, but instead give their support to UKIP. As long as the fourth party is racking up opinion poll ratings in double figures, it will remain more difficult for Mr Miliband to establish the kind of commanding lead and momentum he needs going into 2015.
On issues like law and order and welfare and the impact of immigration on social housing and jobs, Labour also risks losing swathes of working-class voters to UKIP.
For Mr Clegg, last week's results are a reminder that 2015 will be a very different election from 2010, which the Lib Dems fought as the anti-establishment party. Next time, they will not only be a party of government fighting on its record in power, but also the most pro-European voice at a time when the single currency crisis has sent opposition to EU membership soaring.