It’s controversial because one party favors strong voter ID laws which means in the American system the other party must oppose it. A more concrete answer is that the party that favors less restrictions on voting, believes that everyone under the governed regardless of disposition, nationality, age, etc. should have the right to vote. The other party believes that the law says you must be 18 and a citizen to vote in federal elections, and that’s how it should be, and if the law says that then there must be someway to verify that.