In case you forgot, there is still a difference between television and reality.

For one thing, you can't switch reality off. For another, attempts by broadcasters to package political facts only remind the viewer how messy the real world can be. Its resemblance to The Weakest Link is slight.

Politicians, meanwhile, are never too sure about reality. They don't truly know what's going on out there. Trapped on the death march to polling day, they try everything and anything in the hope something will pay off. So everyone is watching TV people talking about the terrific importance of televised political debates? Ignore that, the handlers and advisers remind their candidates, at your peril.

There's a half-truth hidden in all the fuss. Doing horribly badly in a TV debate won't destroy your chances: remember Barack Obama's cataleptic performance in his first encounter with Mitt Romney in 2012. The broadcast circus will be meanwhile be forgotten long before polling day: ITV's Thursday night effort is already on its way to the archives.

As for the "winner", the candidate with all the best reviews, there's the Where Are They Now? problem - otherwise known as the Clegg effect - to consider. The Liberal Democrats had their big moment in the first 2010 debate. Tragically (for them), snap opinion polls have no constitutional validity. Nick Clegg's finest hour turned out to be his only happy time in five long years.

The half-truth survives: what candidates talk about in TV debates matters far less than the fact that debates are talked about incessantly, if only for a few days. That's the prize. ITV achieved an average audience of seven million on Thursday. Even setting aside catch-up TV and coverage from news channels, the figure was well down, interestingly, on the 9.68 million who watched in 2010. Just 16-17 per cent of the electorate caught this week's show. But how many are still oblivious to all the talk of winners and losers?

So pity, if you can, the rivals who couldn't be there on Thursday night. In particular, spare a thought for Jim Murphy, condemned to watch helplessly as Nicola Sturgeon dominated the proceedings and the headlines. Suddenly, in UK parlance, she was becoming a national figure. Henceforth she will be more than just a local phenomenon causing grief for Ed Miliband. Mr Murphy, whose campaign had troubles enough before the TV cameras poked their noses in, is on the list of those "also standing".

Myopically, the Tories are pleased about this, as though their "Vote SNP, Get Labour" claim still has credibility. Since it was always the mirror image of Mr Murphy's favourite line, the assertion doesn't stand scrutiny. Since Ms Sturgeon also made it clear on Thursday that she aims to change UK politics, the idea that she is just Mr Miliband's local difficulty - thereby offering the Tories an opportunity - is risible. The possibility that David Cameron could be "locked out" while Labour are kept on a progressive path is now real.

So who needs Jim Murphy? Such, in essence, is the SNP pitch to voters. It comes with confirmation, if you needed any, that Ms Sturgeon is a politician of substance who can see off three men in suits and a Ukip gargoyle with insouciance. The shadow this slight woman has cast over Mr Murphy's efforts is huge. The oxygen of publicity, as the old cliche runs, has been cut off. It's a practical problem: where can the leader of Labour's Scottish effort find a platform remotely equivalent to the one provided by ITV? He can't.

There's more. Up against three men straight off the Westminster assembly line, with a Ukip City boy pretender oozing poison close at hand, Ms Sturgeon was a reminder to voters across these islands of what politics can be. It needn't depend on London. It needn't be the preserve of middle-class former special advisers apparently bred for the purpose. And it needn't accede, timidly, to their punitive economic consensus.

On the night, many noticed what couldn't be missed: three women asserting, repeatedly, that austerity is a ruinous failure. Ms Sturgeon, Leanne Woods of Plaid Cymru, and Natalie Bennett of the Greens will not be offered the same chance again. That says something about the treatment of women in these debates, and about the closing down of arguments by the men from the big parties. The case against austerity is being squeezed out.

The next encounter, on April 16, will see only Mr Miliband - who demanded debates, after all - facing the three women and the Ukip individual. Mr Cameron has again run a mile. Mr Clegg seems to have concluded that agreeing with Nick is out of fashion. He would rather try to cut his considerable losses than attempt to defend his corner.

The final debate, on April 30, will be a version of normal for those who loathe the transformation of politics in the UK. Just three men from the old, proper parties. Just another outing for the tired BBC Question Time format. Not only will the anti-austerity argument be given short shrift - a remarkable fact, of itself - but Messrs Cameron, Miliband and Clegg will not "interact" directly. They will simple answer questions from another of those TV audiences selected according to a polling company's understanding of reality.

Judging by reactions to Thursday night's event, a good number of viewers are liable to wonder what is going on. Ms Sturgeon's success lay chiefly in saying things that voters, particularly non-Scottish voters, are rarely allowed to hear. She put a proposition before the entire UK: it doesn't have to be this way. The arrogant belief that there is no alternative to some version of austerity - semi-skinned or full, toxic fat - was challenged head on.

Improbably, Mr Murphy would have the Scottish electorate believe he is militant, suddenly, for equality and justice. Labour, in the shape of Ed Balls, has even been claiming that it will "end austerity" with its first budget. If you believe Jonathan Portes of the National Institute of Economic and Social Research - generally a Labour favourite - this could be done by adopting Ms Sturgeon's call for a modest increase in government spending. A Labour refusal to vote for George Osborne's welfare cap wouldn't have hurt.

Mr Murphy's problem is two-fold: whether he has something new to say, and whether anyone is still listening. The Labour man's difficulty did not spring into existence on Thursday night, but it has been exposed to the glare of attention in Scotland and beyond. Those who don't live here don't vote here. Elections are not won in TV studios. But TV's version of reality leaves impressions. This week, the casual viewer in England, Wales and Northern Ireland will have assumed that Ms Sturgeon was the obvious winner of more than a mere TV debate.

London will meanwhile start to get the message. Women with funny accents, straying off the script, outshining the men in suits and taking on received wisdom that hasn't been challenged in decades: it might be the biggest threat Westminster has encountered in a very long time. And it's not, as TV might imagine, for one night only.