Story highlights GOP plans to overhaul the tax system are caught in a year-end legislative thicket

The House Freedom Caucus has gone along with leadership plans until last night

Washington (CNN) House Republicans -- for whom the tax overhaul has gone remarkably smoothly -- got a small taste of what panic feels like Monday night on the floor.

Members of the conservative Freedom Caucus threatened to sink the pro-forma vote to name conferees to the House-Senate tax conference over the length of the short-term government funding bill. They sorted it out, but as one top outside GOP adviser put it to me last night: "It's a good reminder that no matter how well things are going, we're always one troublesome subgroup away from this thing spinning out of control."

The risk: The tax bill is caught in an epic end-of-the year legislative thicket. Until last night, it had been separate and apart from what even optimistic aides describe as a total mess.

That changed Monday night.

We'll see if it's an isolated incident, but the Freedom Caucus, which was remarkably helpful getting the tax bill across the finish line in the House, grabs onto leverage like few other groups in the House. Don't be surprised if this isn't the last threat to the tax bill over an unrelated end-of-the-year issue.

Read More