The Senate will vote as early as today on whether to punish transportation unions in a bid to get Federal Aviation Administration authorization through 2015. The House already passed its version of what negotiators called a bipartisan compromise. But unions object to the deal and have gotten more vocal about it in recent days.

The FAA authorization bill has been held up for months, because House Republicans sought a major change to union election rules in the aviation sector. Specifically, they wanted every worker to count in a union election, even those who didn’t vote; the absent members would count as a “no” vote on the union. Senate Democrats resisted this, even allowing the FAA to shut down for a couple weeks last summer before House Republicans agreed to a short-term extension without the union rider (FAA personnel eventually got back pay).

But the compromise on a long-term extension that emerged had other measures detrimental to labor law, which unions condemned. This includes rules that would make public the vaunted “secret ballot” for those who sign cards wanting a union, and would also make it easier to decertify a union in the instance that a larger and smaller airline merge, as well as limiting the authority of the National Mediation Board, the overseer of rail and airline union laws. I wrote about this previously.

The topline compromise, which raised the threshold of the workers who had to sign cards to trigger a union election from 35% to 50%, didn’t seem like a big deal. Unions wouldn’t be likely to move forward on an election if they didn’t have at least 50% support up front. But these poison pills were obviously what Republicans wanted to salvage what has been a damaging series of confrontations on the FAA authorization bill. They dropped their lead demand of forcing election results to count workers who did not vote in a union election as a No vote. But they picked up all these other nicks in the way union representation works.

Democrats in the House overwhelmingly opposed the compromise bill, but that wasn’t enough to stop it in the Republican-controlled House. The bill also does not include the “passenger’s bill of rights” designed to ensure that planes must not stay on the tarmac for more than three hours before takeoff.

But Senate aides have referred to this bill as a compromise where nobody got everything they wanted, so it looks like it will have the votes to pass there. Labor has continued to blast Senate riders attached to the authorization, but that is the expected outcome today.