Why must "om" be left out?
It doesn't have to be left out, it's optional, you could also say …proberen de verschillende partijen om stemmen te verzamelen.
I included the "om" but it wasn't accepted, so I reported it. I just thought I would ask so I would know quicker, and in the case that it wasn't correct I would know why.
what is the difference between verschillende and andere?
In this sentence andere would translate to other.
Ik begrijp het. Bedankt!