Much of Donald Trump’s State of the Union address was devoted to describing the menaces he claims face America — mainly the menace of scary brown people, but also the menace of socialism.

There has, however, been little talk of one of the most revealing aspects of the SOTU: what Trump said about the menace of America’s historically large government debt.

But wait, you may object — he didn’t say anything about debt. But that’s what was so revealing.

After all, Republicans spent the entire Obama administration warning that America faced a looming crisis unless deficits were drastically reduced. Now that they’re in power, however — and with the deficit surging thanks to a huge tax cut for corporations and the rich — they’ve totally dropped the subject.

According to ABC News, Mick Mulvaney, Trump’s acting chief of staff, explained to GOP members of Congress why debt wouldn’t get a single mention in the SOTU: "Nobody cares."

Republicans never actually cared about debt; they just pretended to be deficit hawks as a way to hamstring President Barack Obama’s agenda. And many centrists have turned out to have a double standard.

There are still two big questions. First, how much should we care about debt? Second, will the deficit scolds suddenly get vocal again if and when Democrats regain power?

On the first question: Everything we know about fiscal policy says that it’s a mistake to focus on deficit reduction when unemployment is high and interest rates are low, as they were when the fiscal scolds were at their loudest.

The case for worrying about debt is stronger now, given low unemployment. But interest rates are still very low by historical standards — so low that we needn’t fear that debt will snowball, with interest payments blowing up the deficit.

It’s still a bad idea to run up debt for no good reason — say, to provide tax breaks that corporations just use to buy back their own stock, which is, of course, what the GOP did. But borrowing at ultralow interest rates to pay for investments in the future — infrastructure, of course, but also things like nutrition and health care for the young, who are the workers of tomorrow — is very defensible.

Which brings us to the question of double standards.

You don’t have to agree with everything in proposals for a "Green New Deal" to acknowledge that it’s very much an investment program, not a mere giveaway. So it has been very dismaying to see how much commentary on these proposals either demands an immediate, detailed explanation of how Democrats would pay for their ideas, or dismisses the whole thing as impractical.

I’m not saying that Democrats should completely ignore the fiscal implications of their actions. Really big spending plans, especially if they don’t clearly involve investment — for example, a major expansion of federal health spending — will have to be paid for with new taxes. But if and when Democrats are in a position to make policy, they should be ambitious, and not let the deficit scolds scare them into thinking small.

Paul Krugman (@PaulKrugman) is a columnist for The New York Times.