Pendel- seems misleading to me because it suggests a regular alternation, as if they were predictably blue one election and red the next. Schaukel- has the same problem, and also that it's not quite the right sense of swing, I agree.
I don't like Zitter- either, because of the unwanted connotations of fear.
And I really don't like Wechselwähler-, because as Mattes says, it's not the individual voters who are undecided, for the most part; it's just that they're lucky (or unlucky, given the deluge of ads) enough to live in a state where there are about the same number of each, so that the race is really competitive, the outcome is too close to call in advance.
To tweak the numbers in #17 a little further, I might guess it's more like
blue state: 53% D, 42% R, 5% undecided/other
red state: 66% R, 31% D, 3% undecided/other
swing state: 47.5% R, 46.5% D (or vice versa), 4% undecided/other
I suppose you could just say Wechselstaaten, but wouldn't that make people think of Wechselwähler?
I suppose 'unpredictable' is the general idea, but it's not really quite the same concept. As I understand it, the point is that those few states have the power to swing an entire election, that is, tilt or tip it to the point where one or the other side wins. In that sense I wonder if something like (um)kippen might be thinkable. Is there any other verb for the action of being das Zünglein an der Waage?
tilt - ... 2: (vt) ... b: (fig) (argument) kippen;
to tilt the balance of power towards/against sb - das Kräftegleichgewicht zugunsten/zuungunsten von jdm verschieben
tip³ - 1: (vt) ... to tip the balance - (fig) den Ausschlag geben
That's on the right track, but would you say 'ausschlaggebende Staaten'? Less of a mouthful anyway than 'kräftegleichgewichtverschiebende' ... \-:
Failing any other bright ideas, I tend to agree with those who are leaning toward something like umschwenken, Wankel-, schwanken. Unfortunately I don't find many existing compounds in the dictionary with any of those as the first part. Would any of those really work, or is there anything else in that general direction?
That is, I think the underlying sense is related not to schaukeln as much as to some of these (again Pons-Collins):
swing - 1: (n) a: ... (fig, Pol) (Meinungs)umschwung ...
2: (vt) ... c: (= influence) (election, decision, voters) beeinflussen; (opinion) umschlagen lassen; (person) umstimmen, herumkriegen (inf);
his speech swung the decision in our favour - seine Rede ließ die Entscheidung zu unseren Gunsten ausfallen;
what swung it for me was the fact that ... (inf) - was dann letzten Endes den Ausschlag gegeben hat, war, dass ...;
to swing it (so that ...) (inf) - es so drehen /or/ deichseln (inf) (, dass ...);
he managed to swing it in our favour - es gelang ihm, es zu unseren Gunsten zu drehen;
he managed to swing the deal (inf) - er hat das Geschäft gemacht (inf)
Does any of that give anyone any ideas?
(By the way, if any of you know anyone in a swing state, now is the time to e-mail them and beg them to vote. Especially in Ohio. Turnout is probably going to be the key. *gnagnagna*)