Let me explain.

America came out of the Great Depression with a pretty effective financial safety net, based on a fundamental quid pro quo: the government stood ready to rescue banks if they got in trouble, but only on the condition that those banks accept regulation of the risks they were allowed to take.

Image Paul Krugman Credit... Fred R. Conrad/The New York Times

Over time, however, many of the roles traditionally filled by regulated banks were taken over by unregulated institutions  the “shadow banking system,” which relied on complex financial arrangements to bypass those safety regulations.

Now, the shadow banking system is facing the 21st-century equivalent of the wave of bank runs that swept America in the early 1930s. And the government is rushing in to help, with hundreds of billions from the Federal Reserve, and hundreds of billions more from government-sponsored institutions like Fannie Mae, Freddie Mac and the Federal Home Loan Banks.

Given the risks to the economy if the financial system melts down, this rescue mission is justified. But you don’t have to be an economic radical, or even a vocal reformer like Representative Barney Frank, the chairman of the House Financial Services Committee, to see that what’s happening now is the quid without the quo.

Last week Robert Rubin, the former Treasury secretary, declared that Mr. Frank is right about the need for expanded regulation. Mr. Rubin put it clearly: If Wall Street companies can count on being rescued like banks, then they need to be regulated like banks.

But will that logic prevail politically?

Not if Mr. McCain makes it to the White House. His chief economic adviser is former Senator Phil Gramm, a fervent advocate of financial deregulation. In fact, I’d argue that aside from Alan Greenspan, nobody did as much as Mr. Gramm to make this crisis possible.

Both Democrats, by contrast, are running more or less populist campaigns. But at least so far, neither Democrat has made a clear commitment to financial reform.