A party’s political platform sets out their core values, but more specifically their concrete goals for the upcoming term. As a body, they stand, as members of that party, for certain proposals. These are not “side issues.” They are central issues.
In some years party platforms are broader than in other years. Increasingly I have seen in my adult life the Democratic Party in particular becoming committed to The Left among their membership, whereas the Republicans are sitll, in their platform, moderate by comparison. (The most Conservative elements of the RP do not dominate, is what I mean.)
So if one identifies with the contemporary Democratic Party one will be identifying with many core values which directly oppose central, priority Catholic doctrine.
Now, individual politicians are not bound by every goal of a party’s platform. However, the party “machine’s” support of that candidate (with voiced and concrete enthusiasm) will very much depend on how conformed that candidate is to the party’s current platform. Dare to be a maverick, and you will be marginalized, and your chances of election affected.