Are Senate Dems and the White House about to create another minor headache for themselves on the left?

I’m told that unions are “frustrated” with both over the big compromise Senate Dems reached with House Republicans on the Federal Aviation Administration reauthorization bill — which unions believe will make organizing harder for hundreds of thousands of railway and airline workers.

But unions are resigned: They expect that Senate Dems will pass the bill tonight, I’m told.

“We’re frustrated that the White House was not more engaged in this,” says Shane Larson, the legislative director of the Communications Workers of America, which along with the International Association of Machinists is leading the battle against the deal.

“The Senate leadership and the White House have left us behind,” adds Rich Michalski, vice president of the Internional Association of Machinists, which is also fighting the deal. “This will negatively impact over 260,000 railroad and rail transportation workers.”

This was one of labor’s top priorities for 2012, though it’s unclear how much of labor is currently united against the emerging deal.

House Republicans had previously tried last fall to insert a union busting provision into the FAA reauthorization bill. But Senate Dems stood firm against it, and the White House threatened to veto it, getting the House GOP to back down and negotiate a temporary extension. This raised labor’s hopes that the White House and Dems would again stand firm when the longer-term deal was negotiated.

It’s not to be. Late last year, Harry Reid unveiled a compromise he’d reached with House Republicans that would do away with the union-busting provision. In exchange the deal would raise the threshold required for triggering a union election from 35 percent worker interest in a union to 50 percent. After examining the deal, unions concluded it could be disastrous for labor, and pilloried Senate Dems for selling out on the deal.

Union officials tried frantically to get Senate Dems to reconsider, I’m told, but couldn’t get traction. They also tried to get the White House to engage, Larson tells me, but that didn’t work, either.

“They’ve responded, they’ve said they understand our concerns and realize why we are upset about it.,” Larson said of the White House, adding that White House officials didn’t push Senate Dems for changes.

Larson says unions are angry in part because the compromise didn’t codify something into law making it impossible for the union-busting piece sought by the House GOP — which would count no-shows as No votes against unionization — to be revived later.

It’s unclear how much the White House is to blame for the current mess. The piece the White House originally threatened to veto is gone from the legislation; and it’s unclear whether unions pressed for another veto threat. Still, if and when this passes, it will cause some disappointment in at least some quarters of the labor movement at a time when Dems had hoped to renew union enthusiasm heading into 2012.