The Catholic vote matters and both parties know it. That's why the debate over John Kerry's (search) support for abortion rights and his desire to seek and receive communion (search) looms as a potentially significant political and theological fault line this election year.

The Vatican (search) has ruled that Catholic politicians are morally obligated to oppose abortion, and Kerry's abortion (search) stance has led some bishops to declare they will deny him communion.

Others, such as the head of the Washington D.C. Diocese (search), Cardinal Theodore McCarrick said he wouldn't go that far, but he would prefer that Kerry not seek communion.

But Catholic Democrats say the church should not politicize communion.

"Is it standing up for principles or is it using the holy sacrament, the holy Eucharist, the body of Christ as a political tool if someone casts a public vote that they may not agree with?" asked Rep. Bart Stupak (search), D-Mich.

Catholics represent one-quarter of the electorate, and are an even higher share in swing states like Ohio, Pennsylvania and Michigan. Because of their electoral significance, Kerry will likely step gingerly around the abortion and communion controversy as the campaign continues.

Click here to watch a report by Fox News Channel's Major Garrett.