WASHINGTON — While lawmakers raising concerns about a potential merger is nothing new, Rep. Barney Frank added a unique twist to his letter asking the Federal Reserve Board to take more time to review the Capital One-ING Direct transaction: the potential for systemic risk.

The Massachusetts Democrat said that because the merger would create the fifth-largest bank by deposits, "care should be taken to thoroughly examine the impact of this purchase with respect to the consolidation of banking assets."

What remains unclear is whether the letter will force the Fed to hold hearings or delay the merger.

The central bank had appeared on track to approve the transaction despite protests from some consumer groups who cited fair-lending concerns. Some observers said Thursday the Fed may at least decide to hold hearings as a result of Frank's intervention.

"It does create a set of new questions for the Fed," said Karen Shaw Petrou, managing director of Federal Financial Analytics in Washington. "This is a seemingly routine transaction that is challenged on the size criterion as a leading indicator of the complexity of transactions like this going forward. They may well have hearings."

Still, most said it was unlikely the Fed would scrap the deal. While the central bank has held hearings on merger transactions in the past, it has rarely scuttled them altogether barring egregious fair-lending or Community Reinvestment Act violations. The last Fed denial of a merger came on Dec. 23, 2002, when the central bank rejected Illini Corp.'s proposed acquisition of Illinois Community Bancorp.

Ernest Patrikis, a former New York Fed official and a lawyer at White & Case LLP, said size alone shouldn't be a factor in denying an application.

The systemic issue is "already there," he said.

"It's just adding to size. Is big bad by definition?" he said. "I hope we don't have that philosophy. The issues should be the normal issues."

The Fed evaluates a merger application based on the management of the acquirer, the financials of the deal, and its impact on the community.

Patrikis noted that if the central bank has concerns, it generally makes them known privately early on in the process, effectively forcing an acquirer to rework or withdraw an application.

"Denials are rare because people don't do transactions that get denied," he said. "You don't spend the time or money. If you don't think you have a very good shot — you don't do it."

McLean, Va.-based Capital One in July brought to market a $2 billion common stock offering to help pay for the $9 billion cash-and-stock purchase of ING Direct. The deal would make Capital One the fifth-largest U.S. commercial bank by deposits, with about $200 billion. The transaction includes $80 billion in deposits, $41 billion in mortgages, and $30 billion in securities.

Petrou said if the Fed does hold hearings or delays the merger it could have a big impact on other potential large mergers and acquisitions.

"This is more important for the precedent it sets for large M&A," she said. "If the Fed will begin to intervene based on concentration issues — which this would do, it makes them a bigger player in a single market — that's a very big antitrust concern that has generally not troubled the Fed."

A Fed spokeswoman said the bank had received 94 letters on the merger. The deadline for comments is Monday.

Capital One expects to close the transaction by early 2012 and have to pay a $270 million termination fee if it falls through.

Earlier this month, Capital One agreed to pay $2.6 billion for $30 billion in credit cards from HSBC Holdings PLC, giving it a channel to invest its new deposits.