Further background here. Democrats and Republicans have been tussling over this bill for a year now, with the key flashpoint being language aimed at preventing transportation workers from forming unions. In the end, Democratic leaders agreed with Republicans on a new measure that largely accomplishes the same anti-union goals -- and labor officials are steamed.
It's unclear how steep the political ramifications will be for Democrats who supported the final package -- including President Obama. But unions will continue to make the point that this is not the year to undermine their priorities if, as an incumbent, you want to rely on union organizing power.