A newly-published article in Politics and Religion examines whether or not the religious affiliation of members of Congress were associated with a likelihood to vote either for or against an Iraq War funding bill.
They find that Catholics and members of minority religions were less likely to vote to fund the Iraq War, even after controlling for other possible factors like gender, race, previous military service, district partisanship, and individual partisanship and ideology. Mormons were less likely to vote for funding when controlling for only for ideology but not partisanship. Protestants were no more or less likely to support Iraq War funding after accounting for the effects of partisanship and ideology.
The bottom-line message seems to be that representatives who are Catholics or are affiliated with minority religions are more likely to deviate from their party in foreign policy measures in Congress, but Mainline or Evangelical Protestants are not.