It costs 30 cents to make a $1 coin, but the Fed purchases it for face value — and the U.S. Treasury pockets the difference. In 2010, the Mint put about 400 million $1 coins into circulation, which means the government made a profit of about $280 million.

The backlog, which was first reported by National Public Radio, has prompted some members of Congress to consider revising the 2005 law.

Rep. Jared Polis, a Colorado Democrat, introduced a bill in July that would allow the government to stop issuing new coins if stockpiles exceeded one year's worth of demand.

"We shouldn't continue to spend money minting coins until it's shown that the public actually has an interest in using them," Polis said. "When they were new, they were collector's items, but now billions of dollars of coins are sitting in storage space at taxpayer expense due to a lack of interest.

"We're not saying put an end to the program altogether," he said. "Just that we should put a hold on producing more until there is a demand for the coins."

The Baltimore branch of the Federal Reserve is a plain, brick building on South Sharp Street. The coins are stored in a basement warehouse partitioned with metal fences and monitored by dozens of security cameras.

Visitors — as well as the bank's managers — must pass through a guarded airlock before they can get to the money. Inside the vault, they are accompanied by two minders.

The last line of defense: A pair of ordinary padlocks hanging on a metal gate just outside the vault.

Inside, coin bags are piled into row after row of shelves. In addition to the presidential coins, the bank also stores Susan B. Anthony dollar coins, which the Mint is no longer producing, and also the gold-colored coins that feature Sacagawea, the Shoshone Indian who guided Lewis and Clark on their 19th century expedition to the Pacific Northwest.

Beck, the Fed's regional executive, said the size of the stash can vary by season — ebbing a bit during summer months, possibly as increased travel drives an overall demand for cash. But it is collectors, according to the Fed, that are the biggest customers.

In an effort to put more of the coins into circulation, the Mint encourages people to purchase them directly on the Internet. But the program was limited after officials realized consumers were using credit cards to buy thousands of dollars of coins in order to accumulate frequent-flier miles, and then immediately returning the coins to the bank.

Now, consumers must use wire transfers, checks or money orders to buy coins online.

Smeallie, with the Dollar Coin Alliance, speculated that people probably wouldn't mind making the switch to coins as much as they might think.

Given the choice, he said, people will opt for what they're most comfortable with. And the $1 coin, he suggested, hasn't been given much of a chance.

He compared it to the 2009 transition from analog to digital television, which required people with older TVs to update their sets with digital converters.

"Everyone was so afraid that there was going to be this hue and cry," he said. "In the end, it wasn't that big a deal."