Sources: Senate Democrats crafting new 'tax extender' bill

The Tea Party has supported upstarts that have already unseated Sens. Lisa Murkowski (R-Alaska) and Bob Bennett (R-Utah). And some Republican senators that aren't even up for reelection this year are tacking further to the right in their positions to keep the Tea Party from supporting a candidate in their home state.

This means Republicans who might have tolerated tax increases in the last extender package might oppose them in the new bill, giving Democrats a tough challenge in locating the right payfors to offset the cost of the legislation.

The new extender bill is also supposed to increase taxes for the oil-spill liability fund, and lawmakers could increase the levy further to make up for the possible shortfall. But Republicans might even deem a tax hike on that fund too risky and oppose it, sources say.

Senate Democrats need the support of at least one Republican to pass the extender bill. 

They are expected to try and attach the package to the small-business bill, but failing to do so means the upcoming debate on the Bush-era tax cuts might also include the fate of extenders, several sources have told The Hill.

- This story was updated at 4:25.