When Lebanon’s bankers turned to DLA Piper and Patton Boggs for help last year, they hoped the lobbying powerhouses could burnish the image of a $127 billion industry tainted by allegations of money laundering for terrorists and drug dealers.

Instead, two Lebanese financial ­institutions last week were hit with U.S. Treasury Depart­ment sanctions, each designated as a "primary money laundering concern" for purportedly aiding Hezbollah, a Lebanese political party that the United States considers a terrorist organization. The classification sets the stage for regulators to cut off the two exchange houses from the U.S. financial system.