If they had hoped the radically changed landscape of British politics would prove a mirage, then both Labour and the Conservatives will be disappointed. Last night's TV debate – with Nick Clegg repeating the accomplished performance of the week before – means the two largest parties now have to accept that they are in a new kind of contest from now till polling day. Any hope that normal service would be resumed has gone.

For Labour, that should prompt a radical conclusion. A government in power for 13 years cannot hope to win an election that is now all about change. Instead its best hope surely has to be to maximise its core vote to prevent a collapse into the low 20s on percentage points, a defeat even more absolute than that of 1983.

That could prompt a shift in message, one that is less about reassuring the one-time New Labour voters of middle England – too many of those are gone now – and more about bringing back home the Labour base. It will mean an emphasis on those aspects of the manifesto that have so far barely got a mention, reminding lower-income voters of Labour's promise to introduce a living wage or to cap the interest rates charged by credit card companies. Such a campaign would be utterly different from the last three Labour has fought, but the game is different now. It is, realistically, no longer about eking out a majority but losing as few seats as possible. What few resources the party has – and they are few – should go to on-the-ground organisation, ensuring that Labour gets its voters to the polls. This is one area where the two "old parties" have an advantage over the Lib Dems, who in many places lack the grassroots machinery to exploit their current surge in support.

The Tories might be tempted by a similar approach, giving up on winning over the golden, swing voters of middle England now seduced by Clegg and seeking to bring out their own core in the hope that it might just be enough. But, given our electoral system, it won't be. It wouldn't give the Conservatives anything like a majority. Instead they have to aim for the same prize they sought before Cleggmania, winning enough new support to sweep to victory.

That means taking on the Lib Dems on the ground they have made their own in the last week, insisting that it is the Conservatives who offer change. That may once have seemed straightforward for David Cameron, but Clegg has made it a whole lot more difficult. For the next two weeks, Cameron has to convey the same freshness and urgency as Clegg, all the while suggesting he is the more substantial vehicle for change. There are some signs of encouragement for him in the details of the post-debate polling. Populus for the Times found that Clegg's high scores of a week ago – for example on having the right ideas to take the country forward – fell by some 16 points yesterday. Inevitably the Lib Dem's novelty value is fading, allowing Cameron to reassert himself as the agent of change.

As for the Lib Dems themselves, they have to keep on doing what they're doing, though Clegg has a challenge of his own. He needs to show he has "another gear", in the words of one seasoned strategist. People have liked what they have seen so far, but he needs to show them there's more. He is in a wrestling match now with David Cameron, both trying to grab the mantle of change – a contest in which Gordon Brown can only look on.