Re: good margin
"[...] to agree, support, and vote for a party."
What a voter believes they will get by voting for a particular party is open to debate. Manifesto promises are just that - promises that can be discarded, postponed, or re-interpreted once they are in power. Sometimes that is because they don't see the practical difficulties until later.
For the bigger parties - the local MP that the voter enables to be elected may be of a minority faction that has quite contrary ideas. Should that minority faction gain executive power then voters will rue the day they voted that way.
There used to be a time when our local candidates or party activists would knock on doors. For the last few decades round here the most one gets is a leaflet headlining some of their "promises" in a PR biased style.
I always try to dig deeper - and it is surprising how often an apparently anodyne party candidate turns out to have some personal incompatible ideas.
Biting the hand that feeds IT
