This is the year that will probably see Ukip leader Nigel Farage and Scottish National party hero Alex Salmond stride grinning into the heart of the very Westminster establishment they profess to despise.

Taking up his seat on the green benches of opposition, Ukip’s newest honourable member might even find himself squashed beside Caroline Lucas of the Greens, whose party split the Labour vote just enough to deny the new prime minister Ed Miliband a majority.

And perhaps to the right of Salmond sits Boris Johnson, the newbie MP for Uxbridge and now clear favourite to win the Conservative leadership contest following the resignation of a defeated David Cameron.

Nick Clegg and a couple of lonely Lib Dems loiter at the back, despondent at losing their space on the government side.

That is just one fantasy parliament – among the most likely, if you believe the pollsters and the bookies – out of all the manifold combinations for the outcome of the election in just over four months’ time.

Tory or Labour majority, Tory or Labour minority, Lab-Lib or Tory-Lib coalition, even Lab-SNP or Tory-Ukip coalition. Any of these are possible at a time when the previously inconceivable option of a multi-party rainbow alliance or Tory-Labour grand coalition has been mooted if Ukip pulled off a miracle.

The point is that not even the most seasoned Westminster watchers feel confident enough to call the election for one side or the other.

Ed Miliband and David Cameron: just 66% of electors express an intention to vote for one of their parties. Photograph: AFP/Getty Images

And while clearly either Cameron or Miliband will find themselves walking into No 10 on 8 May, it may be hard to shake off the feeling that the victory for either man is one of accident rather than overwhelming popular support and ingenious political strategising.

A major theme, therefore, of 2015 is going to be the weakness of the two major parties and how far the minor challengers can inflict damage on them.

Both Labour and the Conservatives are going to be desperate to cast the election as a two-horse race, in which only one of their leaders gets the prize of premiership.

But in reality, they are both going to be jostled all the way along a crowded electoral course by Ukip, the SNP, the Greens and even the deeply unpopular Lib Dems, who hold dozens of key marginals that could decide the outcome. It just depends which of the two key contenders is slowed down more by the other riders.

It is a deeply risky situation for the two main parties that, with so short a time before polling day, this is set to be the year of their lowest popularity since the polling company Ipsos MORI began collecting records in 1978.

In that year, known for its industrial disputes and the winter of discontent, 91% of people still identified with either Labour or the Conservatives.

In contrast, just 66% of us today express an intention to vote for the parties of either Ed Miliband or David Cameron – the only two really feasible candidates to be the next prime minister. This is lower even than 1981, when Labour’s support was hollowed out by defectors to the newly formed Social Democratic party (SDP).

With a similar turnout to last time, only around a quarter of the voting-age population will probably have backed the winning party on election day.

Academics such as Dr Stephen Fisher of Oxford University still predict Labour and the Conservatives will take around 90% of the seats overall, because of the first-past-the-post system that hands victory to whoever is in the lead in each individual constituency.

However, the minor parties have a huge ability to influence the final result, depending on whose vote they devour the most.

Ukip is obviously causing the most high-profile squeeze on the two traditional leading parties, taking at least three times as many votes from the Conservatives as Labour.

But the impact of the other challengers will also cause nail-biting in the party HQs.

Labour has now identified at least 17 seats where it is at risk of failing to take the constituency because the Green party is chomping through their core leftwing support. Party strategists say Iraq – a decade on – is still an issue that is stopping leftwingers from returning to Labour.

In Scotland, despite the election of a credible new national Labour leader in Jim Murphy and defeat of the yes campaign at the referendum, Miliband could be looking at the loss of more than a dozen of his MPs to the resurgent SNP.

And while the Liberal Democrats’ national share of the vote has been severely hit under the coalition, the party is still clinging to the hope that it may hold on in its established strongholds. With this in mind, senior Lib Dem cabinet ministers are spending hardly any time in Whitehall, in favour of lavishing love on their constituencies.

Polling by former Conservative donor Lord Ashcroft shows that support for the party has actually increased in seats such as Sutton & Cheam in south London and Eastbourne on the south coast.

Facebook Twitter Pinterest Alex Salmond: soon to be seen in Westminster? Photograph: Ken Jack/Demotix/Corbis

In an election where a small number of knife-edge seats are crucial to the overall result, the tiniest swing will make a difference. Strategists estimate a good local candidate can give a 2% boost. So prepare for the contest to get hyper-local over the next few months. Senior Tory and Labour politicians will be obsessing about pot-holes, anti-social behaviour and dog poo on the pavement as well as the macro-issues of the economy, immigration and NHS.

Leaflets from the 2015 slate of candidates have begun to drop through letterboxes and there is an evident trend already. Few and far between among the Tory and Labour specimens display photos of the parliamentary candidates glad-handing voters with their dear leaders in shot. Some have only the tiniest references to their own parties. There is plenty of “normalising” of CVs to stress real-world jobs and working-class roots when in truth they should be catalogues of lives lived in Westminster.

It seems the traditional parties are a liability to the parliamentary hopeful, not an asset – something of which the new crop of MPs will be only too aware.

To return to that fantasy parliament, it is likely to be a boisterous one. If its members end up owing little of their success to the merits of their respective Tory and Labour leaders, the Commons is bound for five years of rebellion.