Generically moderate to conservative swing voters...This isn't too hard.
But the question would be why they'd support Obama, and then Romney... purely because they're swing voters?
In late-2011/early-2012, the economy started to recover a little faster from its stagnation just in time to help Obama, so there may have been a "hey, things are starting to get better, maybe Obama's not so bad after all" thinking that got Obama more support. Now, the economy is still in "2% mode" as it was in 2012 and wages haven't caught up, so there's some disappointment and a "let's give him a chance, maybe I was wrong about 2012" thought line that could net Romney votes.
In 2012, blaming Republicans and GW Bush was still a valid argument for Obama as less than 4 years had elapsed, while Obama owns the economy in 2016.
I can see Romney try to run the "Actually, I was right about everything" line on foreign policy, which despite being oversimplified, would be hard for Democrats to respond to (How can they fit a detailed explanation of the complex geopolitical history behind ISIS and Russia's actions into a 3 minute debate argument, let alone a TV commercial? They can't, so Romney can easily say "It's all Obama's fault because he's weak").
If you think about how most swing voters are, it's not hard to come up with a plethora of scenarios and reasons why they change their votes.
