Sweeping food safety legislation passed by the US Senate on Tuesday but not yet signed into law hit an unexpected roadblock yesterday.

Republicans insistent that expiring Bush-era tax cuts be extended before the Senate’s lame duck session ends later this month said that they would refuse to pass any bills unless the Democratic-controlled Senate first extends the tax cuts and enacts next year’s federal budget.

“While there are other items that might ultimately be worthy of the Senate’s attention, we cannot agree to prioritize any matters above the critical issues of funding the government and preventing a job-killing tax hike,” all 42 Senate Republicans wrote in a December 1st letter to Senate Majority Leader Harry Reid (Democrat, Nevada).

Normally, this would not be a problem for the bill, which extends broad new food-policing powers to the Food and Drug Administration, and which passed the Senate on Tuesday by a vote of 73-25. But after passage, a hitch was discovered: the bill could conceivably be defined to raise taxes, by imposing fees on food importers and on domestic manufacturers whose food is subject to a recall when it is discovered to be tainted. If those fees are defined as taxes, then, under the US Constitution, the bill must first be passed by the House of Representatives. The House passed a food safety bill in July, 2009, but it is different than the Senate version—- meaning that, for the Senate bill that was passed on Tuesday to become law, it must first be voted on by the House, and then again passed by the Senate, before being signed into law by the president. Senate Republicans say such a re-vote is a no-go, unless and until the terms they laid out in their letter to Reid are met.

Stay tuned to this space for the fate of the food safety bill that, earlier this week, seemed to be a signal that Democrats and Republicans in a polarized Washington, D.C., could at least agree on something.