"We have heard reports that certain interests might be pushing to attach such a bill to a 'must pass' measure because they have calculated that a secretive, closed-door, undemocratic process represents their best opportunity to regain access to the U.S. market," they wrote.
"Creating a Federal right to gamble that has never existed in our country's history, and imposing an unprecedented new tax regime on such activity, requires careful deliberation -- not back-room deals or earmarks for special interests," the letter continued.
Reid's office declined to comment on the bill or its prospects in the final weeks of the year.
Still, some sources close to the legislation acknowledged that the broad mix of constituencies pushing for greater government control of an industry that has operated in the shadows, may help it succeed as an attachment to a larger piece of legislation.
"Licensing and regulating online poker is the most effective and responsible way to ensure the online safety of America's children and consumers," said Parry Aftab, an internet privacy and security expert who has testified before Congress on the need for legislative reforms.
"Given the growing popularity of online poker, especially on college campuses, we cannot afford to miss this opportunity to protect America's children and consumers. We must take action now," Aftab said.