Describe an Obama '08 / Romney '12 / Trump '16 voter

[Senator-elect Spark]

..

[Hindsight was 2020]

Someone who likes voting for the party out of power because it makesxthem feel like the smartest person in the room

[Statilius the Epicurean]

Millions of people

[Xing]

Someone who is constantly unhappy with the status quo, and always wants "change."

[Lambsbread]

Tiffany Trump?

[Attorney General, LGC Speaker, and Former PPT Dwarven Dragon]

Someone who normally votes Republican, but was very displeased with Dubya's second term and wanted to send that message through their vote.

[Warren 4 Secretary of Everything]

2/3 of an idiot

[pbrower2a]

Someone scared that 2008 was a replay of 1929, satisfied by 2010 that America was not going into a Second Great Depression, but was satisfied with an economy resembling that of the 1920s. 

Ask again in 2021 about the Obama '08/ Romney '12/Trump '16/Democrat of 2020 voter.

[AN63093]

Millions of people

Yeah, this.  I mean, basically pick any state in the Midwest on the Atlas and click on the Swing map.  There ya go.  There are dozens upon dozens of these counties, and no way to categorize all these people in a few words.

You might as well be asking "Describe someone who votes Republican."  The responses you'll get will be about as equally unhelpful.  Maybe that was the point of the OP, I dunno.

[Cory]

Donald Trump.

[True Federalist (진정한 연방 주의자)]

Not me. I was a McCain '08 / Johnson '12 / Clinton '16 voter. (If I'd been in a competitive state, I'd have been Obama '12.)

[Bojack Horseman]

The opinion columnist from my local paper. Was fairly liberal until 2009 when he went full-right-wing baths!t insane.

[mileslunn]

I would put them into a few categories.  By far the largest swing was in the Midwest so I suspect you would have the greatest number of them found there.  A couple of others as mentioned below

1.  Normally votes Republican, but was so angry at how bad things were in 2008 so voted Democrat that one time, but that went back to their traditional voting pattern and also hoped Obama would be truly different than most Democrats and transformational.

2.  Traditional conservative Democrats who Obama was too liberal so they finally started voting Republican even though philosophically they were already leaning that way.  A lot of the traditional White Southern Democrat from 2000 onwards trended this way and so each election a certain portion of that group swung over.  Middle Tennessee, Coal fields of Kentucky, West Virginia are examples where you each election have a certain percentage of traditional Democrats who flip GOP.

3.  Person struggling and thus wants to punish whomever is in power and since none have managed to fix the problems they are the type who will continuously vote for the opposition.

[Orthogonian Society Treasurer]

Somebody from Indiana.

[Badger]

Grumps

[BlueSwan]

2/3 of an idiot

I wouldn't call Romney voters idiots, but I guess voting for Trump more than makes up for it, so I'll go along with the 2/3.