They rely on polls. Very long and specific polls. This cycle a lot of us are looking at some seriously wrong polling. But that's how larger campaigns and politicians make decisions.

Marijuana legalization probably isn't polled unless it's on the ballot.

But politicians have rational incentive to move towards what is clearly popular. 60% is a landslide. There are definitely 'Mavericks' who hold strong to their beliefs or independence, but for the most part those who win follow their constituents rather than lead.