One of the biggest-selling points for debit cards has been that you couldn't spend more than you had in your bank account, making them a good alternative to racking up debt on a credit card.

The sales pitch worked. Consumers now make one-third of their in-store purchases with a debit card — up from 21 percent six years ago, according to the American Bankers Association, an industry trade group.

But as many users have discovered, the debit card carries the same problem as its credit card cousin — you can spend over your limit, causing yourself all kinds of financial havoc.

The practice of approving a check or debit transaction when the account has insufficient funds is an intentional effort to drive up revenues, according to a new study by the Center for Responsible Lending, a nonprofit, nonpartisan research and policy organization.

"Banks should protect customers' funds, not plunder them with high fees and harmful practices," said Eric Halperin, director of CRL's Washington office.

Earlier this year, CRL published a report that found that debit card purchases at POS (point of sale) terminals and withdrawals at ATM machines triggered 46 percent of overdrafts. The rate for paper checks was just 27 percent of overdrafts.

In CRL's most recent report, "Out of Balance," the consumer advocacy group found that debit cards are the largest source of account overdraft fees for banks and credit unions. Debit card POS and ATM overdrafts cost consumers $7.8 billion in 2006, which represented 45 percent of the $17.5 billion the financial institutions received in overdraft fees last year.

Many banks and credit unions automatically enroll customers for overdraft protection programs when they open a checking account, according to CRL. When an overdraft occurs, the financial institution will automatically advance the funds but charge the customer a fee. For the privilege of a short-term loan to cover the shortfall, the customer is charged on average $34, according to CRL's data. The fee kicks in even if the overdraft is for a few dollars.

In hopes of reducing overdraft fees charged to consumers, the CRL is supporting legislation sponsored by Rep. Carolyn Maloney, D-N.Y., and Rep. Barney Frank, D-Mass., that would, among other things, require banks and credit unions to warn their customers before authorizing an electronic overdraft. The House recently held a hearing on the legislation.

"I've always said that banks have the right to make money for what can be a valuable service, but consumers also have a right to information they need to make an informed decision," said Maloney, chair of the House Financial Institutions subcommittee.

Bankers argue that overdraft protection is not only something consumers want but need, considering many do overdraw their checking accounts.

"It helps to avoid embarrassment, inconvenience, merchant fees and other adverse consequences of having a check bounce or a transaction denied," Nessa Feddis, senior federal counsel for the American Bankers Association, said in recent testimony before a House subcommittee.

Feddis said it's the responsibility of the customers to keep track of their money because they are in a better position to know their actual balance. Only they know about the most recent automatic payments they have authorized and their debit card transactions.

Still, why won't the institutions warn customers about possible overdrafts? "Current technology makes real-time notification of overdrafts cost-prohibitive," Feddis said.

Interpreted another way: Financial institutions don't want to change the status quo because they make good and easy money off their own customers' mistakes or irresponsibility.

The fact is, Halperin said, that the technology exists and should be installed to deny a transaction if there isn't enough money in a person's account.

On this issue, everyone should be held accountable. Consumers have many ways to check what they have in their accounts before using their debit cards, including going online or using the automated phone system at their bank to get an update. To cover themselves when mistakes happen, they can link their checking account to a source of backup funds — a savings account or line of credit. Or they could just opt out of having overdraft protection.

And financial institutions should restrain their fee frenzy and stop routinely allowing consumers to withdraw funds that exceed their account levels, only to slam them with overdraft charges.

Michelle Singletary writes for The Washington Post. Contact her at singletarym@washpost.com.