That clever Ally Bank ad, the one where a boy is denied a toy truck because of a "limited-time offer," omits a fact that would interest most viewers.

You, the taxpayer, are propping up Ally, the bank that's so good at making fun of other banks. And it looks like Ally's parent, GMAC Financial Services, will ask for more money soon.

The government, which already has invested $12.5 billion in GMAC, may have to inject $5.6 billion more to keep the lender afloat. GMAC apparently hasn't been able to convince private investors to supply the capital it needs.

The additional investment shouldn't be made until we can answer a simple question: Why?

GMAC is the 14th-largest U.S. bank by assets, but it doesn't fall in the too-big-to-fail category. It's not as critical to the global financial system as, say, a Citigroup or a Bear Stearns.

It is, however, considered critical to two of the government's non-bank wards, General Motors and Chrysler. Their dealers depend on GMAC for financing, and the thinking in Washington is that no other bank would want to be in that business. If that's true, the failure of GMAC would leave GM and Chrysler virtually unable to sell cars.

It's a reasonable assumption, says Rebecca Lindland, an automotive analyst at IHS Global Insight. "Financing is a huge part of the equation when it comes to people buying and leasing vehicles," Lindland says. "If credit is not available, it definitely has a dampening effect on vehicle sales."

She notes that JP Morgan Chase and other banks left the automotive market after getting burned on lease financing. In these

risk-averse times, she doesn't think they will want back in: "It's not a very attractive industry right now. We're coming out of the worst year in history for vehicle sales."

Consider this, though: If banks are skeptical about the profit potential in auto lending, is this a good risk for taxpayers to be taking?

When Uncle Sam gets involved in an industry, risk gets mispriced. A risk-averse bank would probably charge higher interest rates to GM dealers, and require higher credit scores for car buyers. That would hurt GM's sales volume.

"It's like we're going to subsidize the sale of cars for these guys, so we're going to prop up the lending institution," says Glenn MacDonald, the Olin professor of economics and strategy at Washington University.

Consider, too, a couple of fairness issues. CIT Group, another troubled lender, is trying to restructure privately, and may yet file bankruptcy, even as GMAC comes back to the public trough. Essentially, the government is saying that CIT's customers, most of them small businesses and retailers, are less deserving than the auto companies.

GMAC, through Ally Bank, also competes with other banks for insured deposits. In addition to running ads featuring toy cars and ponies, Ally entices consumers with above-market interest rates. That angers the American Bankers Association, which says the government-backed competitor is raising costs for all banks.

Thomas Hough, chief executive of Carrollton Bank in Des Peres, doesn't see GMAC as a direct competitor, but he thinks the government should have an exit strategy. If there's money to be made in auto finance, he says, "I would think the market would respond to that opportunity fairly quickly."

Taxpayers already have a 35 percent stake in GMAC, and that figure apparently will go higher. We involuntary investors know how we got into this commitment, but we would like to know how we are going to get out.