Washington Senator Maria Cantwell was one of only two Democrats voting today to block progress on the Senate's big financial reform bill. (The other was Russ Feingold of Wisconsin.)

There were enough votes in favor of ending debate on the measure for it move along toward a vote—meaning Cantwell's stand ends up being more about principal than practical matters. But what was her stand about, exactly?

In a word: Derivatives.

She simply doesn't feel there are going to be enough checks on buyers and sellers of these complicated financial instruments to prevent another period of "tremendous damage" to the economy.

Cantwell spokesman John Diamond said that just now, with the Senate voting on the overall bill, Cantwell again cast a "no" vote.

He said she'll work to strengthen the derivatives language in conference committee.

UPDATE: The bill—minus Cantwell's vote—has officially passed the Senate.