It's as if the recent sunny, muggy weather has spread into politics; there is a smiling, dazed feel about the coalition, a willingness for them to do well, a pleasure in the sight of politicians being polite about one another. It won't last, says the sceptic. And it means absolutely nothing, adds the cynic. And the sceptic is right; the cynic, wrong.

It does mean something. For far too long, Westminster culture has been much nastier and meaner spirited than the country generally. This nastiness began with the vicious fratricidal wars inside old Labour, then through the savage edge of the Thatcher years. It boiled and suppurated as the Tories tore themselves apart over Europe. It was back in a new form – New Labour, New Nasty – because of the personal vendettas behind the smiley face of Blairism.

And now, at least for a while, it has subsided. The coalition has its tensions, of which more in a moment. Vince Cable and George Osborne are not a mutual admiration society. But overall, we see people striving to find common ground, watching their words, and showing one another a bit of respect. We see something very similar in the Labour leadership contest, whose contenders report a surprising enthusiasm and energy in their local meetings.

This is all to be welcomed because Westminster nastiness was weird, dysfunctional and mostly unnecessary. It put normal people off politics, except as a spectator sport up there with cage wrestling, and it wasted creative energy the country actually needed. Just imagine what New Labour might have done without the huge percentage of time and thought devoted to the Blair-Brown feud; or what Major's government could have been like without the "loonies" and the "bastards". This Westminster underworld was summed up forever by The Thick of It and Malcolm Tucker. Normal people looked and laughed. But they don't like it, and never did.

So the cynic is wrong and politicians should take heed. More politeness and less macho posturing would go a long way to rebuilding interest in politics. But the sceptic, who says this honeymoon cannot last forever, is right too.

Look at the very different spins being put on the public spending cuts by David Cameron and Nick Clegg today. The Tory prime minister emphasised the depth of the problem and, by implication, the sour harshness of the medicine about to be administered. His Liberal Democrat deputy emphasised that these would be sensitive, caring, progressive cuts. Between Cameron's "it's gonna hurt" and Clegg's "but not so much" lies the fissure inside the cosy-sounding phrase "Liberal Conservative".

The truth is that Cameron's message is more realistic than Clegg's. Even as he seeks to appease core Tory interests on capital gains tax and military spending, he knows the axe will fall savagely across the public sector. This will anger and disappoint Lib Dem voters more than Tory ones; can it be long before the likes of Charles Kennedy emerge as anti-coalition spokesmen?

For the time being, at Westminster, the coalition will hold together because it has to. As the polls turn against them, the politicians will have no choice but to try to stay the course through some hard years, and hope that the recovery comes in plenty of time to save them.

But there's the question: to save who? In his Observer interview, Clegg strongly denied that his party would be eaten alive by the Conservatives. It was his job, he argued, to keep the parties distinct. Maybe so, but the likelihood is that the Lib Dems will pay a very high price for their alliance. Who remembers the National Liberals, swallowed by the Conservatives more than half a century ago?

Either the coalition will succeed in its economic programme, and emerge more popular; or it will fail. If it succeeds, voters will reward it. But the face will be the face of the prime minister, Cameron, and the Tories will reap most of the reward. If a Tory-dominated government has won through, voters will be inclined to back them. Why would you vote against the dominant partner in a coalition, in order to support the weaker one?

The bigger the pro-coalition verdict, the larger the majority, and the more of that majority will be Conservative. Would a "coalition" election platform, with distinct party platforms, produce a second hung parliament? This one has been unusual in UK political history, so the answer may well be no. The Lib Dems could therefore face the paradoxical situation that their very success in government produces an election result that makes them unnecessary. Oh dear.

Alternatively, of course, the coalition could fail and have to face the wrath of disappointed voters; who would, presumably, therefore vote for the opposition, which is now Labour, not the Liberal Democrats. Neither outcome looks a happy one for the junior partner.

Clegg's answer to this, the only plausible one, is that he must get a change to the voting system before that election takes place, to ensure the third party is better represented. That's why the timetable for proposals and a referendum on voting reform has taken so much hard bargaining behind the scenes. Without it, the Lib Dems will indeed be destroyed. The Tories, wanting a "proper" majority next time, know it very well.

It follows that the politics leading up to the referendum, the campaign itself, and anything that follows, will be very tough. The referendum must divide the coalition and will be even more contentious at Westminster than the tax rises and spending cuts. For the Lib Dems at least, it's a matter of the party's survival as a serious force. Meanwhile, the Tories will think: if we can scupper voting reform and get our changes to the constituencies through, we can destroy the sandal-wearers for good.

Everything suggests that this confrontation may happen, on a Lib Dem timetable, as early as next year – in other words, at the same time as the worst of the spending cuts are being implemented. Then, I fear, nasty politics will be back. The muttered insults between tribal Tory and Lib Dem politicians will spill out into the open. Labour may even be able to pose as the "clean" party, interested in weightier matters than mere party political positioning.

I'm sure that Clegg and Cameron are entirely sincere in saying they want no election for five years and will try hard to achieve that. It's in their personal interests. It's a lot less clear their parties will feel it's in their interests, or be able to deliver such a long union. So the sceptic who says it can't last may be proved right earlier than most people expect. A message for Labour? Your leadership election is a serious and urgent business. Get on with it fast, and try as hard as you can not to be nasty.