A Dem leadership aide tells me this is likely a temporary setback.
"We are still whipping the rule to ensure that it passes," the aide says. "Liberals are not happy with the tax bill in general. The rule is typically where a lot of members will lodge their protest."
"We always knew that the rule vote would be tough ... we just need more time," the aide added.
If Dems can't be corralled, leaders will have to come to terms with rank and file members on a new rule that wins majority support.
Late Update: Speaker Pelosi is now conferring with her caucus on language for a new rule that could pass. Rules Committee Chairwoman Louise Slaughter (D-NY) tells us that she expects this dispute to be resolved and the vote on the tax cut plan to still be held today, although it may be tonight rather than this afternoon, as originally scheduled. The situation, however, remains fluid.