In a speech to the United Auto Workers just now, Obama defended his decision to bail out the auto industry, lacing into Mitt Romney with withering derision. But this speech was about more than the auto-bailout. It was Obama’s case for reelection.

This speech constituted Obama’s most ambitious effort yet to weave his defense of the auto rescue into the larger contrast he will try to draw between his vision and the “you’re on your own” ideology he will accuse Republicans of representing.

Romney is likely to be a formidable general election opponent, particularly if the recovery doesn’t accelerate. And the voters at stake in the argument over the auto bailout — blue collar whites, independents, struggling swing state voters — are very much up for grabs.

But today’s speech was important: It revealed that the alternate reality Romney has been functioning in throughout the GOP primary is soon going to give way to another reality entirely, a general election reality — and Romney, presuming he will be the nominee, will soon collide with it.

In recent days, that alternate reality has meant that Romney has been speaking to audiences who nod along when he tells them that he got the auto-bailout right, even though he predicted that it would lead to the auto industry’s demise. It has meant speaking to audiences who cheer when Romney makes the convoluted case that things would be better still if we hadn’t pursued the auto-bailout, and that government intervention accomplished nothing more than a giveaway to “union bosses.”

Things will soon change rather abruptly.

In his speech, Obama — without naming Romney — went out of his way to ridicule his justifications for opposing the auto-bailout and his current dissembling about how he got it right. Here’s the key segment:

A key line: “I keep on hearing these same folks talk about values all the time. You want to talk about values? Hard work: That’s a value. Looking out for one another: That’s a value. The idea that we’re all in it together and I’m my brother’s and sister’s keeper: That’s a value.”

Obama used the auto-bailout argument as a jumping off point for his larger case: That Democrats and Republicans have fundamental ideological differences over government’s role in stepping in and protecting working people and ordinary Americans against the depredations and excesses of unfettered free market capitalism, and that this all flows from a difference over moral values and priorities.

Dems shouldn’t get too sanguine about this argument; polls show that public attitudes about these matters are volatile and in flux; the state of the economy could exert a powerful gravitational pull on them. But what you’ve just seen is a preview of how aggressive a bid Obama will make to ensure that the general-election debate over the auto-bailout plays out his way.