The Roman Catholic Church says...
In this context, it must be noted also that a well-formed Christian conscience does not permit one to vote for a political program or an individual law which contradicts the fundamental contents of faith and morals. -- (CDF DOCTRINAL NOTE
on some questions regarding The Participation of Catholics in Political Life, 4)
The Church teaches that, regarding a law permitting abortions, it is "never licit to obey it, or to take part in a propaganda campaign in favor of such a law, or to vote for it" -- (John Paul II, Evangelium Vitae, 73).
"When legislation in favor of the recognition of homosexual unions is proposed for the first time in a legislative assembly, the Catholic lawmaker has a moral duty to express his opposition clearly and publicly and to vote against it. To vote in favor of a law so harmful to the common good is gravely immoral" -- (CDF Considerations regarding Proposals to Give Legal Recognition to Unions between Homosexual Persons, 10).
The Democratic Party says...
We will defend the dignity of all Americans against those who would undermine it. Because we believe in the privacy and equality of women, we stand proudly for a woman's right to choose, consistent with Roe v. Wade, and regardless of her ability to pay. We stand firmly against Republican efforts to undermine that right. At the same time, we strongly support family planning and adoption incentives. Abortion should be safe, legal, and rare. -- (Democratic Party Platform 2004)
We support full inclusion of gay and lesbian families in the life of our nation and seek equal responsibilities, benefits, and protections for these families. In our country, marriage has been defined at the state level for 200 years, and we believe it should continue to be defined there. We repudiate President Bush's divisive effort to politicize the Constitution by pursuing a "Federal Marriage Amendment." Our goal is to bring Americans together, not drive them apart. -- (Democratic Party Platform 2004)
So there you have it. The Catholic Church specifically teaches Catholics not to vote for laws or candidates that permit abortion and gay-marriage. The Democratic Party on the other hand, specifically says that it stands up for a woman's "right" to have an abortion, and for the states to authorize gay-marriage against the will of the people.
Can a good Catholic vote for this Democratic agenda? NO! Not without violating Catholic teaching on social responsibility. Like I said, it is possible for a single Democratic candidate, in a small local election, to oppose the Democratic Party's platform on these positions and get away with it. But on the higher level of national politics, that kind of candidate is virtually impossible to find. The Catholic Church and the Democratic Party are on total opposite ends of the spectrum on these key social issues facing our nation today. The US Catholic Bishops have called upon Catholic voters to change their parties, and not let their parties change them. As it stands right now, the Democratic Party is totally at odds with the conscience of good Catholic voters. It must change. Or else it must not get our votes. To continue to support a party that opposes the Catholic Church on so many levels is a sin to say the least, and certainly NOT the action one would expect from a good Catholic.