It's often said in Washington that Republicans are better in opposition than at governing, and the last two years have borne out this observation with a vengeance. It sure looks like it's been fun being a Republican. The economy was terrible, and the other guys were in charge. And even though it was mostly the Republicans who messed up the economy in the first place, and who started the wars that neither George W Bush nor (probably) Barack Obama will really and truly be able to count in the win column, they knew that they could rely on Americans to forget that over time – and forget Americans did. So all they had to do was sit back and throw darts.

Into the bargain, and probably to the great surprise of many of them, the Tea Party movement erupted out of some Americans' rage at the government and at their fellow citizens who took out mortgages they couldn't quite afford. The media often write about the tension between Tea Party insurgents and establishment Republicans, and it's there. But mostly the movement has been as pennies from heaven for the GOP: you have a bunch of extremists running around comparing the Democratic president to Hitler and Stalin. If they go too far, you can gently denounce them. But mostly, you just let them carry on with their wild analogies, which work their way into the civic bloodstream but for which you do not get blamed. It's been a great racket.

But now the times they are a-changing. Having taken control of the House of Representatives as of tomorrow, Republicans now have to govern. They have to do things like make a budget. And not just a fake budget, like in a campaign. A real budget, that adds up, more or less. They have to negotiate with a Senate still in Democratic hands over the final shape of appropriations to the various federal agencies. All that sounds suspiciously like hard work. And Washington Republicans, for all their thumpety-thump rhetoric about hard work and personal initiative and so on, are largely lazy and unserious people. They won't do the work, and in two years, it will show.

How can I say that? Alas, recent history bears it out. When I say lazy I don't mean that they fail to arise from bed. They manage that. I mean intellectually lazy. And yes, unserious. Let's look at the last three Republican presidents, going back to 1980. In that time Republicans have been screaming about the budget deficit. So what did they actually do to fix it? Ronald Reagan opened up a gaping hole, which was somewhat repaired from its worst point by the time he left office but was still far larger than that of Jimmy Carter, his predecessor. On the whole, Reagan lost America $81bn. Think that's a lot? George HW Bush cost the country $135bn. Think that's a lot? His son cost us – get ready – $632bn. Bill Clinton, meanwhile, made us $526bn.

Most liberals call this hypocrisy, and it is that. But it's something even worse than hypocrisy. It's complete and utter lack of seriousness about governing. Hypocrisy is, at the end of the day, just an allegation about character. But that combined $848bn they've added to the deficit: that's real money, pal. And they don't do a thing about it, really. They yell and scream that it's all the Democrats' fault. A little of it is. But most of it is the fault of the massive tax cuts Republicans have pushed through, which have left revenues and expenditures wildly out of balance.

Failures to cut spending on the domestic front largely reflect the wishes of the American people, who call themselves conservative in theory, but who, in practice, want to see the government spend money on entitlement benefits, education, environmental protection and so on. Republicans secretly know this and respond to it. They had the run of every branch of government in the early 2000s, and what did they do with it? Increased spending and expanded Medicare!

They're not serious people. They're great at theatre. We all know that. They will open the new session of the House of Representatives over which they now preside with a public reading of the full text of the constitution, taking turns. (I wonder who gets to read the bit about slaves counting as three-fifths of a person?) That's excellent PR. And they're matchless at their little rhetorical ornamentations, like "death tax" (estate taxes) and "death panels" (which did not exist).

But running the country? They've shown almost no aptitude for it for many years. The reason is simple and was imperishably expressed by the scholar Alan Wolfe in an essay he wrote four years ago: "Conservatives cannot govern well for the same reason that vegetarians cannot prepare a world-class boeuf bourguignon: if you believe that what you are called upon to do is wrong, you are not likely to do it very well."

Obama has not on the whole been a commanding and decisive leader so far. And his fate is still lashed chiefly to the economy, and if it's still tottering in two years' time, he will suffer for it. So I can't say with confidence yet how he'll be positioned as November 2012 approaches. But I can say this. It's highly likely that after watching Republicans for the next two years, a majority of Americans will conclude that Obama is the only grownup in the room.