I've heard a lot of Republicans on here say they "know" Trump will win in 2020; so tell me, how do you know this?
In 1892 the economy was good and Benjamin Harrison lost re-election.
In 1912 the economy was good and William Howard Taft lost re-election.
In 1968 the economy was good, President Johnson didn't run again, but his party still lost.
but think about this: it wasn't the 3rd parties that changed the election. It was the lousy performance of the incumbent president that PROMPTED the rise of a 3rd party. The 3rd party was a reflection of events, not the cause of events.