The crudest definition of big-tent politics is generally ascribed to Lyndon Johnson. Better to have a rival inside the tent relieving himself in an outward direction, said LBJ, than outside aiming (not his word) in. Would such a thought ever cross the mind of Gordon Brown?

He would only be human if it did. The sound politicians like best is the sound of agreement. If they cannot convince by argument they will resort to flattery, persuasion, bullying, bribery or even - though all concerned should know better - an appeal to patriotism. More often than not they invoke consensus, as though this is a supreme political virtue, next only to godliness.

Is it? Of recent Prime Ministers only Margaret Thatcher had the honesty, or the arrogance, to deny any interest in a consensual approach to government. She took it for granted that politics is essentially adversarial. As even some of her own followers confessed, she was an authentic class warrior for whom the only good opponent was an opponent crushed underfoot.

Her style is out of fashion these days. At Holyrood, given the circumstance of minority government, everyone is in the consensus business. Alex Salmond and the SNP have no choice in the matter. Their opponents embrace the spirit of alleged co-operation, with any number of caveats, because such, so they persuade themselves, is what the voters want. It is the political equivalent of decorum.

Again: is it? Wouldn't it be more honest for opposition MSPs to admit that they cannot stand Mr Salmond, deplore everything he stands for and intend to do everything in their power to obstruct his government in an effort to throw it, and him, out of office at the next opportunity? That sort of confession would at least bear a relationship with the truth.

You can guess, though, how the First Minister would respond. He would shake his head sadly. He would observe that such behaviour scarcely serves the greater good of the country. He would ask how it could possibly be in the interest of "the people". Then he would hint that the people themselves will surely pass judgment on those who put "narrow party interests" above the welfare of the nation.

Depending on your point of view, it is either an old but neat trick, or a recognition that politics need not be, should not be, entirely self-serving. If Mr Salmond invites a suddenly ubiquitous Henry McLeish to serve on his broadcasting commission, for example, either conclusion is available.

You could say, cynically, that a former Labour First Minister is simply being suborned and exploited by the Nationalists, and that he should know better. Or you could say that Mr Salmond is simply making the best use, for the common good, of a predecessor's experience and ability.

There is, in fact, a third possibility. Political scientists used to call it convergence. Back in the 1960s, in the aftermath of Butskellism, it was all but taken for granted that ideological differences were eroding, that factions within the political class were becoming indistinguishable. Party labels, it was said, were ceasing to mean much. Tories had accepted the welfare state settlement; Labour had yielded to economic realities, so called. Neither party, when in government, disputed the need for nuclear weapons.

Thatcher upset that apple cart, more than somewhat, but she seems increasingly like an aberration, or at least an atypical catalyst, even within the history of Conservatism. Her lasting impact, in fact, was on the Labour Party, in the equally anomalous figure of Tony Blair, the disappearing statesman, now faded from public memory like a strange dream.

What remains is David Cameron, a Tory aping Blair, content to be accused of "trashing" the Thatcher legacy. What remains is Gordon Brown, a Labour leader ostentatiously refusing to ape Blair, who this week made a point of expressing his admiration for Thatcher and her maniacal "convictions".

If there is a war of ideas going on between these representatives of the 21st-century political class, it is difficult to say where the front line might lie. What's the real difference between them? Does it still exist?

If it survives, the Prime Minister, for one, will be disappointed. This week, addressing the National Council of Voluntary Organisations, he said: "I believe this is the wrong time in history for politics as usual; the wrong time for empty partisan posturing which focuses on what divides us, faced with common challenges; the wrong time for continuing to treat citizens simply as members of contending groups as if there was no scope for common ground."

Perhaps Mr Brown has taken LBJ's advice to heart, but he would have us believe otherwise. This, said the no-spin spin, is new politics, a tent big enough for a three-ring circus and every talented act around. The Prime Minister wants to see talent from any and every quarter "co-operating across party lines to work together with patriotic purpose". He deplores that "empty partisan posturing", those "sterile divisions and archaic battles".

In fact, Mr Brown's diagnosis is a bold one, since it appears to disparage his own entire career, hitherto, among many others. "The depths of our new concerns," he said this week, "cannot be met by the shallowness of an old-style politics."

Already a few recruits have rallied to the cause. John Bercow, Tory MP for Buckingham, has agreed to head a standing commission on services for children with communication disabilities. Patrick Mercer, a Tory sacked by Mr Cameron for certain remarks on racism, is to advise on security. Matthew Taylor, a LibDem, is to have a say on rural housing.

Meanwhile, Digby Jones, the CBI stalwart, has agreed to bounce for Brown. Johan Eliasch, sports goods magnate, environmental philanthropist and former Tory deputy treasurer, has deserted his party in protest at a "lurch to the right" (presumably taking £2.6m in loans with him) in order to advise the government on forestry. The path once followed by Lord Sainsbury and other refugees from the SDP is becoming well worn as Labour - or rather the Prime Minister - draws on people with business, military and voluntary-sector experience. None elected, as it happens.

Meanwhile, the first citizens' jury begins work on the problems facing the young. A "citizens' summit" designed to produce a "British statement of values" is planned. And the Prime Minister still has in mind an all-party Speaker's Conference to examine, without partisanship, a possible lowering of the voting age, weekend voting, the position of women and ethnic minorities in the political system, and the like.

Voters are expected to approve. Mr Brown would not be wasting his breath if he thought otherwise. The belief, a common one, is that a lack of engagement has brought politics in Britain to a crisis and that what voters cannot stand, above all, is "bickering".

Perhaps so. I've a suspicion, though, that the electorate recoils more from the thought that "they're all the same". Peering into the Prime Minister's tent, indeed, it is hard not to suspect the electorate might be on to something, that new politics means, in effect, no politics. Old-style shallowness - so twentieth century - was at least supposed to suggest that old-style arguments were worth having, that some things were worth fighting for. Can we get a consensus on consensus? Not from me.