America used to be a country that built for the future. Sometimes the government built directly: Public projects, from the Erie Canal to the Interstate Highway System, provided the backbone for economic growth. Sometimes it provided incentives to the private sector, like land grants to spur railroad construction. Either way, there was broad support for spending that would make us richer.

But nowadays we simply won’t invest, even when the need is obvious and the timing couldn’t be better. And don’t tell me that the problem is “political dysfunction” or some other weasel phrase that diffuses the blame. Our inability to invest doesn’t reflect something wrong with “Washington”; it reflects the destructive ideology that has taken over the Republican Party.

Some background: More than seven years have passed since the housing bubble burst, and ever since, America has been awash in savings — or more accurately, desired savings — with nowhere to go. Borrowing to buy homes has recovered a bit, but remains low. Corporations are earning huge profits, but are reluctant to invest in the face of weak consumer demand, so they’re accumulating cash or buying back their own stock. Banks are holding almost $2.7 trillion in excess reserves — funds they could lend out, but choose instead to leave idle.

And the mismatch between desired saving and the willingness to invest has kept the economy depressed. Remember, your spending is my income and my spending is your income, so if everyone tries to spend less at the same time, everyone’s income falls.

There’s an obvious policy response to this situation: public investment. We have huge infrastructure needs, especially in water and transportation, and the federal government can borrow incredibly cheaply — in fact, interest rates on inflation-protected bonds have been negative much of the time (they’re currently just 0.4 percent). So borrowing to build roads, repair sewers and more seems like a no-brainer. But what has actually happened is the reverse. After briefly rising after the Obama stimulus went into effect, public construction spending has plunged. Why?

In a direct sense, much of the fall in public investment reflects the fiscal troubles of state and local governments, which account for the great bulk of public investment.

These governments generally must, by law, balance their budgets, but they saw revenues plunge and some expenses rise in a depressed economy. So they delayed or canceled a lot of construction to save cash.

Yet this didn’t have to happen. The federal government could easily have provided aid to the states to help them spend — in fact, the stimulus bill included such aid, which was one main reason public investment briefly increased. But once the G.O.P. took control of the House, any chance of more money for infrastructure vanished. Once in a while Republicans would talk about wanting to spend more, but they blocked every Obama administration initiative.

And it’s all about ideology, an overwhelming hostility to government spending of any kind. This hostility began as an attack on social programs, especially those that aid the poor, but over time it has broadened into opposition to any kind of spending, no matter how necessary and no matter what the state of the economy.

You can get a sense of this ideology at work in some of the documents produced by House Republicans under the leadership of Paul Ryan, the chairman of the Budget Committee. For example, a 2011 manifesto titled “Spend Less, Owe Less, Grow the Economy” called for sharp spending cuts even in the face of high unemployment, and dismissed as “Keynesian” the notion that “decreasing government outlays for infrastructure lessens government investment.” (I thought that was just arithmetic, but what do I know?) Or take a Wall Street Journal editorial from the same year titled “The Great Misallocators,” asserting that any money the government spends diverts resources away from the private sector, which would always make better use of those resources.

Never mind that the economic models underlying such assertions have failed dramatically in practice, that the people who say such things have been predicting runaway inflation and soaring interest rates year after year and keep being wrong; these aren’t the kind of people who reconsider their views in the light of evidence. Never mind the obvious point that the private sector doesn’t and won’t supply most kinds of infrastructure, from local roads to sewer systems; such distinctions have been lost amid the chants of private sector good, government bad.

And the result, as I said, is that America has turned its back on its own history. We need public investment; at a time of very low interest rates, we could easily afford it. But build we won’t.