There must be a million married partners who sometimes rue not the day they married, but the day they missed the chance to lay down a harder bargain for matrimony. In one of the most brilliant passages of sustained repartee in English comedy, the Restoration playwright William Congreve places his courting couple in The Way of the World in just such a prenuptial exchange as the lady (Millamant) lays down her list.

“Trifles,” she insists, such as: “... liberty to pay and receive visits to and from whom I please; to write and receive letters, without interrogatories or wry faces on your part; to wear what I please; ... to have no obligation upon me to converse with wits that I don't like, because they are your acquaintance, or to be intimate with fools, because they may be your relations. Come to dinner when I please, dine in my dressing-room when I'm out of humour, without giving a reason. To have my closet inviolate; to be sole empress of my tea-table, which you must never presume to approach without first asking leave. And lastly, wherever I am, you shall always knock at the door before you come in. These articles subscribed, if I continue to endure you a little longer, I may by degrees dwindle into a wife.”

When the Opposition woos the voters with a view to marriage, it too should take a view about how hard a bargain it dare drive before the wedding bells sound. The courtship - the general election - can happen in such a whirl. Much may have been said that was carelessly generous. Unrealistic impressions may have arisen; hopes engendered that are bound to be disappointed. Expectations may have been fanned by never being properly disavowed. Anxious to get the nation down the aisle, an Opposition is apt (as often as not by silence) to shrink from securing the national consent to some unwelcome likelihoods about the post-honeymoon years. The strategy may too easily hint at big, vague, swoon-inducing promises - then hope to let the nation down gently once the knot is tied.

And indeed, when the polls are neck-and-neck and the result on a knife edge, only a false prospectus may swing it for the challenger.

But today, some two years before the likely date for a general election, can't the Conservative Party dare to be more confident than that? My gut feeling is that almost nothing can stop the present Government losing. The Tories do not need to promise much to clinch it. David Cameron needs only to be not Gordon Brown. He can therefore afford to risk the truth: that we may have to face a rough time at first under a Conservative Government.

Advice to the present Prime Minister is that if he would but accept that he's heading for defeat, this could liberate him to run an honest administration for the two years remaining and do some needed but unpopular things in the national interest. There is a corollary: if Mr Cameron will but accept that he's heading for easy victory, this could liberate his party to be an honest Opposition for the final two years, and say some hard, unpopular but needed things in the national interest.

Of course it's always tempting when a relationship is going well to do all you can to keep it that way, and a party's relationship with the voters is no exception. Politicians are nervous souls, ever ready to see disaster round the corner; and today is somehow never the moment to say anything upsetting to voters. But Tories should remind themselves of their catchphrase about Mr Brown's decade as Chancellor: that he missed the chance to fix the roof when the sun was shining. For the Conservative Party the sun is now shining. This is the time to spend a little popularity on honest repairs to the Tory prospectus.

Is the undertaking to match Labour's spending plans sustainable? Now could be the moment - as Mr Brown describes looming “world” economic difficulties - for the Tory leadership to revisit that undertaking in the light of changed circumstances, and ask whether Labour can afford its own spending plans.

Would the voters really be too appalled to hear the Opposition say there may have to be a squeeze? Is there a better time than now, when public servants, like the police, feel so intense a hatred towards the Government, for Conservatives to warn the public sector that there will be no bonanza under a Tory administration either? And, resisting temptation to join the motorists' bandwagon, why not - advancing behind the cover provided by fury at this Government - signal that there are no Tory promises to halt or reverse fuel tax rises?

Turning his attention from swing voters to the Tory core, Prime Minister Cameron may sooner than he thinks wish he had lowered expectations in this quarter too. Why not remind supporters that there are limits to what can be achieved in rebalancing Britain's relationship with the European Union? Riding high as he is, he could today ride out the disappointment of the Europhobe element in his party. The same goes for defence spending, where Liam Fox's rhetoric is in danger of raising false hopes.

On fox-hunting too, unrealistic expectations are best deflated early: the promise simply of a free vote and government time to seek a compromise will anger some with any mention of compromise, but may save Mr Cameron from cries of betrayal after taking the reins of government, when he could do without the distraction.

And if there are unpopular big projects on which the Tories prefer to keep an open mind (Heathrow's third runway? Nuclear power?) I'd argue for signalling this now.

Otherwise, when? As they hum and hah their way to postponing a decision on a change of leadership until it is too late, Labour is playing straight into Tory hands. With any luck the Opposition can look forward to two years mauling a Government that, with the media wind against it, appears unable to do anything right. In such benign circumstances, why scrat around for every extra quarter percentage point lead?

Better to go into a general election, head held high, with the betrayals got in early, and an electorate unable later to complain that they hadn't known what they were getting, or The Guardian to shriek that, once elected, the wicked Tories were ripping off their compassionate Conservative mask. As a desperate Prime Minister cold-calls the voters at dawn, the next two years are looking like a very good time for the Tories to bury bad news.

It is more than liberating, it is positively energising, to go into government knowing that the country knows what to expect and voted for it. I started - so I'll finish - with another favourite quote from Restoration comedy. Before marriage (wrote Richard Sheridan in The Rivals) Hope “paints many a gaudy scene”, but “let us deny its pencil colours too bright to be lasting”.

Call me a spoilsport, but this is the time to purge the Tory palette of its rosiest tints.

Times Recommends

• It's time to take on the gangsters of the sea
• Déjà vu: six steps that make up a great panic
• Biographies that cast long shadows