The law of God is that Marriage is forever, till death do us part.
There is no such thing then as 'divorce' in Catholic law.
The law of the State however has 'divorce' in that the state will recognise the legal ending of marriage.
For Catholics however, you cannot separate the two. Any Catholic, who votes for something that inevitably leads to sin (in this case adultery) will have to take some responsibility in these sins. The same applies to abortion or any other state law that accommodates sin against God.
It is then 'a sin' to vote for divorce.