The horrible economic news released yesterday represents the first empirical challenge to the set of messages delivered so potently at the Democratic National Convention — messages that began President Obama’s climb in the polls to undeniable front-runner status now.

Those messages were: He was handed an impossible job to do. He did the best he could, especially with those pesky Republicans trying to stop him. Now you should give him another four years because he tried so hard, he meant so well — and things are improving, slowly but surely.

It’s the “things are improving” storyline that got hammered yesterday.

First came word that the already anemic level of economic growth in the second quarter (April-June) was actually worse than first reported. Rather than the economy growing by a tortoise-like 1.7 percent, it actually dragged itself forward by a near-comatose 1.25 percent.

This means there was a dramatic slowdown in the rate of growth in the first half of this year. The economy went from 4 percent growth in the last quarter of 2011 to 2 percent growth (first quarter 2012) and then to 1.25 percent. It’s as though a foot landed on the brake and began pressing down harder and harder.

Worse still was the drop in durable-goods orders (cars, household appliances, heavy machinery, airplanes). They fell by 13.4 percent, a fall to levels not seen since the depth of the recession in 2009.

Some economists say this isn’t as catastrophic as it looks because it involves a drop in orders of airplanes, a notoriously volatile subsection of the economy. JP Morgan put it mordantly yesterday morning: “Durable goods headline looks like an F, details look like a D.”

These complicated numbers won’t move the presidential race by themselves. We know that because the horrific jobs report for August came out on the heels of the Democratic convention, and didn’t make a whit’s difference.

That’s due in part to the fact that the president’s powerful Front Four — the mainstream media — didn’t take the tiny number of jobs created and the terrible drop in the number of people looking for work and trumpet those two facts for days, as they might’ve under other circumstances.

But it’s also due to the fact that macroeconomic statistics don’t stand alone. For the vast majority of people, percentages of decline in this figure or disappointing growth in that number are a jumble.

Such stats, taken in isolation, confuse more than they reveal. Is a person who doesn’t follow politics or economics closely supposed to know that a growth rate of 4 percent is cause for celebration but 2 percent is very problematic?

Explaining all this is the job of the politicians who seek our vote. Barack Obama has made his case — he’s tried; he needs more than one term; what he’s done saved us from the pit and set us on the right course even if we’re going too slowly.

It’s the best, maybe the only, argument he can make for himself, and he’s done it well.

Now it’s up to Mitt Romney and Paul Ryan to harness this most recent data as part of an overall argument about how his own government’s numbers reveal how bad a president Obama has been — not how he’s a nice guy who doesn’t know what he’s doing, which seems to be Romney’s default (and wildly ineffective) line of argument.

They need to argue that Obama is wrong, and that the data prove he’s wrong.

They have a fresh opportunity to argue that he has not done his best; that what he’s done has not turned things around; that there is every reason to think we are now headed backward; that we’re not on the right track — and if the country sticks with him, things are going to get worse, and in a hurry.

Romney keeps talking about all this more in sorrow than in anger. That’s not working. If this doesn’t make Romney mad, why should he expect it to make anybody on the fence angry enough to act on his behalf and fire his foe?