I meant to link to this yesterday but forgot. Dave Weigel has a genuinely interesting Slate piece about TSA's security procedures that sheds some light on the partisan origins of today's backlash:
According to Tom Blank, the deputy administrator of TSA in the post-9/11 years, the agency was always cognizant of how unpopular scanning could be.
"I used to sit around and look at these images, dial them back, and ask myself how do I take this to Capitol Hill and not be thrown out on my head?" said Blank. "When [Bush's second TSA administrator] Kip Hawley came in [in 2005], he changed that. He saw the politics of it and deep-sixed the program. He deep-sixed it. It got revived after the Christmas bomber."
The point about how full-body scanning got restarted is essential—it was the Obama administration picking up an idea that Republicans had cooled on. Republicans accused the administration of degrading security by dialing back war-on-terror prosecutions in the name of human rights; the response was a security measure that would affect all travelers randomly.
There's much more at the link. Basically, Dave suggests that the libertarian/conservative wing of the Republican Party has long been skeptical of TSA's powers, but felt constrained to shut up and support President Bush while he was in power. So they didn't say much of anything. Now, though, there's nothing stopping them: "The return of [their mistrust and fear of TSA] is a lagging reaction to the fact that Republicans no longer have to toe the party line on homeland security. They can say what they think, which is that the state can't be trusted." Interesting stuff.