Leading article: Mr Cameron and three-party politics

If there is one single feature that distinguishes a US election from a British one, it is the expectation that candidates will show due humility. While an American candidate, for President or any other office, courts defeat the moment he so much as hints that victory might elude him, here the opposite applies. A candidate for elected office must never get above himself and presume victory, however propitious the opinion polls and however confident he might be. Accusations of hubris have the capacity to drag him right back down.

Nick Clegg has sailed dangerously close to the wind in recent days, venturing to dictate – so it has sometimes seemed – not only the terms on which the Liberal Democrats might join a coalition in the event of a hung parliament, but the make-up of that coalition as well. But the Conservative leader, David Cameron, has gone further. He has at times, and increasingly, appeared almost to snatch power before polling day has even dawned.

Hence the force of Mr Clegg's accusation that Mr Cameron has been "measuring up the curtains for No 10" before the country has voted. Hence also, perhaps, the humbler tone of Mr Cameron's address to London Citizens yesterday and the Conservatives' final party election broadcast, which shows Mr Cameron emphatically telling his audience that "the public are the masters; we are the servants".

Even now, though, he is persisting down two avenues that may look more like blind alleys by the time this election is over. Despite the likely discrepancy between the popular vote and the number of seats won – a discrepancy already striking at the election five years ago – Mr Cameron rejects any change in the voting system, insisting that first-past-the-post makes for clear results and a more effective way, not just of electing a government, but ousting one that has lost voters' confidence.

At the weekend, the Conservative leader also made known his aversion to coalition government, suggesting that if his party fell short of an overall majority, he would prefer to form a minority government and try to legislate through ad hoc parliamentary votes. In so doing, it appeared, he would keep the prospect of another election essentially as a threat, in the hope that any party that forced a vote of no confidence would have their comeuppance at the ballot box, handing the Conservatives the overall majority they had lacked.

Either way, the Conservatives – and Mr Cameron – seem stuck in a two-party mentality that ignores the new dynamics of British politics. Whatever the latest vicissitudes of the polls, there are now three parties in electoral contention, and an overall majority of the sort first-past-the-post used to yield may become more difficult for any party to achieve. It seems unrealistic, if not outright arrogant – Mr Clegg's words – for one party to reject in advance electoral reforms that would give more voters a voice.

It seems ill-advised, at the very least, for any party to assume victory, or even king-maker status, given the continuing closeness of the polls. But excessive modesty entails risks, too. A party leader might appear unready for power, or uncertain what to do with it. Or he might come across as reluctant to level with the voters about unpopular measures he has in mind – which has been a running complaint through this campaign. Voters well understand that they will be called upon to tighten their belts still further, given the parlous state of the public finances. Yet most party leaders have naturally preferred to stress what they will spare.

Political expediency dictates that we will probably be none the wiser by polling day. Equally, the new circumstances that will prevail on 7 May, whatever they are, could force some rapid rethinking of previously fixed positions. Were that to include Mr Cameron's rejection of coalitions and electoral reform, so much the better.