Quite a few readers have written in recent days with questions about the Republicans' ability to filibuster the $410 billion spending bill that's currently on the Senate floor, which is expected to come to a final vote late tonight or tomorrow morning.

Can you filibuster this spending bill? Yes -- because it's not a budget resolution, which is a non-binding document that sets general revenue levels for the next fiscal year. The $410 billion measure is what congressional types call "omnibus appropriations," meaning that it sets overall spending levels for various governmental departments from now until October, when the 2010 fiscal year begins.

So when you read about Mary Landrieu (LA), Ben Nelson (NE), and other Democratic centrist senators who are bridling at the high spending levels in President Obama's budget, it's important to remember that they're referring to the non-binding, filibuster-proof document that will likely come to a vote by mid-April.

Democrats can afford to lose as many as eight of their own senators on that vote, while still passing a budget with 50 votes and Vice President Joe Biden as the tie-breaker. The party can also use "budget reconciliation" rules that would allow for filibuster-proof passage of health care, climate change, or even student loan bills later in the year, provided that such legislation achieve a demonstrable reduction in the deficit.

The total savings can be small; for instance, last year the Democrats used reconciliation to pass a student-loan bill that saved $75 million, which is small potatoes compared with the overall budget but achieved meaningful reform for anyone attending college. No decision on reconciliation has been made yet, but it's safe to say that the debate is heating up.