[count-your-change]

“If that’s the case, a Catholic, regardless of his party affiliation, shouldn't’t be voting for such a person.”

“Shouldn't” is a bit different from “can't”. The big question is whether any Catholic voter will change their vote because of this statement.

[annalex]

“Can” applies to statements of physical ability. “Should” in American usage applies to statements of advisability, such as this one.

Learn English. Great language.