The pendulum long ago stopped being a standard feature of our clocks, so perhaps its disappearance from our politics was also a matter of time. But for simplicity’s sake, or maybe out of habit, the battle for control of Westminster is still treated as a contest between red and blue camps.

Last week’s autumn statement, for example, was drafted by George Osborne with the purpose of steering politics back to the binary question of whether Conservatives or Labour should be trusted to run the economy. But next May’s general election will not be decided by a simple migration of voters between Labour and the Tories. Opinion polls show tiny volumes of switching between the big two. The longest and noisiest front in the battle is where least terrain is changing hands.

Instead the outcome will depend on the relative success Ed Miliband and David Cameron have in staunching the flow of supporters to smaller parties. The fate of marginal constituencies will be decided by the compound effect of those movements. Labour makes gains where Nigel Farage has done a thorough job wooing disgruntled Tories. The Tories can retain vulnerable seats if former Liberal Democrats endorse the Greens. No pendulum’s arc can describe a trajectory where incumbent Tory MPs need lefty votes to go further left and Labour challengers need Conservatives to turn harder to the right.

Some of the weirdest dynamics are inside the coalition. To have any chance of increasing his total tally of seats, Cameron needs to make gains at the expense of the Lib Dems in the south-west. Yet he would also like the Lib Dems to stage some kind of recovery in the Midlands, where their collapse is boosting Miliband’s chances. But Nick Clegg has written off seats where his party is facing a Labour challenge, and is concentrating scarce resources shoring up defences against Cameron’s candidates. That is why he went to Penzance and not parliament on the day of the autumn statement.

In the following days Osborne derided the Lib Dems for mixing their economic messages; and Danny Alexander, Osborne’s Treasury sidekick, accused the Tories of callousness and pandering to Ukip. Like spandex-clad wrestlers, the coalition partners charge at each other with real rivalry but simulated aggression. A convention has emerged from years of “quad” meetings between Cameron, Osborne, Clegg and Alexander to manage coalition business: each side knows there will be partisan attacks and asks only for notification that one is coming.

Cameron must now persuade liberal voters that he is not a conduit for Ukippery, and persuade Ukip voters that he is no liberal. Labour revelled in that contradiction before finding itself in a similar quandary. As Miliband’s share of the vote has shrunk he too has had to go fishing in the murky waters of Ukip support. That involves acknowledging resentment of levels of immigration in terms that many on the left find distasteful. It is another supposed betrayal – along with compromises on budget discipline, welfare and capitalism in general – fuelling the defection of perpetual protesters to the Greens.

Then there is Scotland, where opinion polls foretell an SNP tsunami sweeping aside Labour. Scottish Labour MPs say things are bad, but not quite that bad. Their malaise is compounded by a leadership vacuum, but that will be filled on Saturday, probably by Jim Murphy. His enemies depict him as a Westminster carpetbagger and holder of heretical “Blairite” views on nuclear defence and foreign interventions. His supporters point to a record of campaign effectiveness and potential appeal beyond the tribe. He entered parliament in 1997 by poaching a Tory seat, and has turned it into a Labour stronghold.

That is a rare feat among frontbench politicians. The preferred route is selection in a safe seat. As a result, most senior figures in the main parties , including Miliband and Cameron, have little experience of winning by persuasion. They have manoeuvred through the ranks of their parties where factional squabbles feel more pronounced from the inside than they look from the outside. Their advancements owe more to deals brokered among the like-minded than changing minds.

Cameron’s and Miliband’s leaderships are defined by failure to proselytise. In Cameron’s case it is the failure to convert the Conservative party to a “modernising” course in opposition, followed by failure to persuade enough voters in 2010 that his party was no longer the nasty creature of folk memory. So he was denied a majority.

In Miliband’s case it is the failure to turn theoretical diagnosis of flaws in Britain’s economy into contagious enthusiasm for Labour. The premise of his leadership was that the global financial crisis proved the case for a new balance of power between government and markets. Miliband was going to restore the nation’s appetite for the politics of solidarity and equality. But if he wants to impress voters with visions of a new economy he must convince them to trust him with the existing one. He has done neither.

Labour and Tories will fight the election under leaders with proven inability to reach voters across the main political faultline. They are twisted into grotesque contortions, cringing to one set of voters in ways that repel their neighbours; applying rhetorical balms that soothe one flank while inflaming the other; pretending to despise views for some audiences and indulging them elsewhere. The spectacle makes it too easy for smaller parties to say their bigger rivals are oh-so-alike in lack of principles.

Miliband and Cameron will win back some support in the election campaign by mobilising fear of the other man’s accidental victory. One of them will be prime minister; but which it is could come down to as marginal a calculation as whether Green supporters hate the idea of a Tory government more than Ukip supporters recoil at a Labour one. There is no majority to be had that way, and no mandate for government either. The main parties are running out of time to rediscover the politics of persuasion. The pendulum may be broken; the clock is still ticking.