Last Saturday’s debate on the Stupak anti-abortion amendment to the House health reform bill provides an X-ray of the complex interplay between region and religion within the Democratic coalition.

When it comes to social issues, religion matters among Democrats, not just Republicans. Of the 64 Democrats who supported the Stupak anti-abortion amendment, 35 (55 percent) are Catholic. In the Democratic caucus, by contrast, Catholics make up only 38 percent of the total. Put differently: 36 percent of House Democrats who are Catholic supported the amendment, versus 18 percent of non-Catholic House Democrats.

Although we lack district-by-district breakdowns of religious affiliation, data from the Pew Forum on Religion and Public Life indicates that a majority of the Catholics who supported the Stupak amendment hail from states with above-average Catholic populations, and the ethnic composition of their districts suggests that they have large numbers of Catholic constituents.

Region matters as well. Of the 29 non-Catholic Democrats who voted for the Stupak amendment, 20 hail from conservative southern districts, and two from heavily Catholic districts in Texas and New Mexico. The remaining seven represent conservative-leaning districts in Indiana, Utah, West Virginia, Minnesota, North Dakota, and Ohio.

No Democrats from the western-tier states of Washington and Oregon or the mid-Atlantic states of New York, New Jersey, and Maryland voted for the amendment; of the 21 New England Democrats, only four--all Catholic--supported it.

Although the religious and regional balance of the Democratic Party has changed in recent decades, the need to manage a diverse coalition has not. It’s a reality that Democrats angry about compromises that congressional leaders make would do well to keep in mind.

Click here to read Alan Wolfe's argument that the vote actually illustrates the waning influence of Catholicism in politics.