FEMA says provision in appropriations act does not roll back flood insurance rate hikes.
Mississippi-filed lawsuit now seen as last chance to stop January rate hikes.
Congress is currently deflecting intense pressure to roll back virtually all the increases in flood insurance premiums imposed in 2012.
Amendment passed Tuesday would bar the rate increases by preventing FEMA from using federal funds to implement Sec. 207.
The amendment passed 281-146, and the appropriations bill was later approved by the full House and sent to the Senate.