Free checking began as a privilege. Once it spread, customers felt entitled. Eventually, it became a commodity, as most banks felt they needed to offer it. Soon, people took it for granted.

But now, free checking may be an endangered species.

Banks are feeling heat from all sides. This week, President Obama moved to limit the size and activities of the biggest institutions. Last week, he proposed a tax to recover bailout funds.

The biggest impact on checking accounts, however, is likely to come from new regulations governing overdraft protection. Starting in July, banks will need explicit permission from customers before allowing them to use their debit cards to spend more than they have in their bank accounts on a one-time purchase. Similar restrictions will apply to A.T.M. withdrawals.

Banks earn billions in overdraft fees, money that helps pay for free checking.

A chunk of that revenue will disappear when some consumers elect not to sign up for the opportunity to spend more than they have. This week, Bank of America said that $160 million in overdraft fee revenue had already disappeared, because of changes it made in its policies ahead of the new federal rules.