The result was a financial industry that was hugely profitable as long as housing prices were going up  finance accounted for more than a third of total U.S. profits as the bubble was inflating  but was brought to the edge of collapse once the bubble burst. It took government aid on an immense scale, and the promise of even more aid if needed, to pull the industry back from the brink.

And here’s the thing: Since that aid came with few strings  in particular, no major banks were nationalized even though some clearly wouldn’t have survived without government help  there’s every incentive for bankers to engage in a repeat performance. After all, it’s now clear that they’re living in a heads-they-win, tails-taxpayers-lose world.

The test for reform, then, is whether it reduces bankers’ incentives and ability to concentrate risk going forward.

Transparency is part of the answer. Before the crisis, hardly anyone realized just how much risk the banks were taking on. More disclosure, especially with regard to complex financial derivatives, would clearly help.

Beyond that, an important aspect of reform should be new rules limiting bank leverage. I’ll be delving into proposed legislation in future columns, but here’s what I can say about the financial reform bill the House passed  with zero Republican votes  last month: Its limits on leverage look O.K. Not great, but O.K. It would, however, be all too easy for those rules to get weakened to the point where they wouldn’t do the job. A few tweaks in the fine print and banks would be free to play the same game all over again.

And reform really should take on the financial industry’s compensation practices. If Congress can’t legislate away the financial rewards for excessive risk-taking, it can at least try to tax them.

Let me conclude with a political note. The main reason for reform is to serve the nation. If we don’t get major financial reform now, we’re laying the foundations for the next crisis. But there are also political reasons to act.

For there’s a populist rage building in this country, and President Obama’s kid-gloves treatment of the bankers has put Democrats on the wrong side of this rage. If Congressional Democrats don’t take a tough line with the banks in the months ahead, they will pay a big price in November.